Introduction
The three Simmon brothers began producing enlargers in the mid-30s and are still in business today (as OmegaSatter). The original company was Simmon Brothers, and the greek Ω symbol was the logo; but over the years the symbol became the name of the product. Enlargers by Simmon Brothers are almost always referred to as "Omega" enlargers.
Omega enlargers typically follow a reliable name scheme: the letter designates the format size of the enlarger (i.e. the largest negative the enlarger can accept); the following number indiciates the model generation. A "B" size enlarger is larger than an "A", a "D" is larger than both, etc. Typically the higher the number, the more recent the model. E.g. a B-22 is more recent than a B-3.
The letter formats are: "A" (35mm), "B" (2-¼-square), "C" (3-¼-square), "D" (4x5), "E" (5x7) and "F" (8x10).
There are some exceptions, as noted. Marketing departments love to depart from a standard naming scheme, and in some cases a very popular model enlarger (such as the D2) remained in production after more recent machines were discontinued.
I have a few of Instruction Manuals and brochures available for download. 
Quick Comparison
| Model | Mfg | Type | Condenser | Size | Notes |
|---|---|---|---|---|---|
| Basic | 1941-1961 | B&W | Yes | Various | Required auxiliary sets of condensers for different focal lengths |
| Colorhead | 1946-1950s | Color - filter pack | Yes | D series | Similar to the Basic, but with a color filter drawer. Took oddball, oversized filters. |
| Color Filter Drawer | 1946-1950s | Color - filter pack | Yes | D series | Similar to the Colorhead, but took standard 5" filters |
| DV | 1950s-onward | B&W | Yes | D series | The long-time standard B&W (no filter drawer) head. Condensers can be shifted inside for different focal lengths |
| D6 Zoom | 1966-1975 | B&W | Yes | Prolab D6 | Very similar to the DV, but the condensers can be repositioned by a knob instead of by physically moving them inside the drawer |
| Chromega | 1959-1969 | Color - filter wheel | No | B and D series | First variable color head. Uses graduated filter wheels to set color |
| Omega-Sphere | 1960-1962 | B&W | No | B and D series | A chopped-down Chromega without the filter wheels (for diffusion) |
| Omega Light | 1951-1960s | B&W | No | B, D and E series | A cold-light (flourescent) head |
| Chromega Halogen | 1968-1969 | Color - filter wheel | No | B series | Pretty much a Chromega but fitted with higher-output halogen lamps |
| Super Chromega | 1969-1970s | Color - filter wheel | No | D series | Used halogen bulbs for brighter output. Also had lamp-saver feature, illuminated dials |
| Chromega B Dichroic | 1974-1979 | Color - dichroic | No | B series | Available on the B-22, B-66 and B600. Uses an external power supply. Modern Photography Test July 1975 |
| Super Chromega C Dichroic | 1972-1980 | Color - dichroic | No | C-67 | Uses slide-rule levers instead of dials. Adapters available for other C series chassis |
| C700 Dichroic | 1979-1980 | Color - dichroic | No | C-700 | Also fit the B-22, B-66 and C-67 chassis |
| C760 Dichroic | 1980-onward | Color - dichroic | No | C760 | Also fit the B-22, B-66, C-67 and C-700 chassis |
| D5500 | 1982-1999 | Color - dichroic | No | D5500 | Computerized instead of mechanical |
| Super Chromega D Dichroic [II] | 1980-onward | Color - dichroic | No | D series | Mechanical (not computerized). The II version is from 1974 on with very minor tweaks |
| Super Chromega E Dichroic | 1980-onward | Color - dichroic | No | E series | Same design as the Super Chromega D Dichroic |
Comments
Basic
A simple condenser head, with no tray for filters. The D-series required various extra condenser sets to match the format.
Colorhead
This is a basic model with a drawer that would accept acetate filters. The filter size, however, is unusual (6-3/8 x 6-3/4"); larger filters must be cut down to fit it.
Color Filter Drawer
Very similar to the colorhead, but it takes standard 5-inch filters.
DV
This basic condenser head is still in production. The front door opens to allow the user to easily shift the position of the supplimentary condensers to match the format.
Zoom
Very similar to the ubiquitous DV, except that the supplimentary lens could be positioned using a knob on the outside of the head, rather than by physically moving the lens itself.
Chromega
The first of Omega's purpose-built color heads. Unlike other heads which may have drawers for optional filters, this head uses a circular filter with graduated density, so the operator can simply turn the wheel and adjust the color accordingly. The wheels in this head are acetate, not dichroic, and are subject to fading and color shifts. This unit also used two 100w lamps (one on each side), and it looks like something Thomas Edison made at Menlo Park.
Omega-Sphere
A Chromega without the color filters, making it a diffusion source instead of a condenser head. It didn't last long.
Omega Light
This is a cold light (i.e. flourescent) head.
Chromega Halogen
A short-lived upgrade to the original Chromega for B-series enlargers. It used one very-bright halogen bulb instead of the original bulb, but was quickly replaced by the Chromega B Dichroic model.
Super Chromega
An upgrade to the original Chromega, it sported 150w bulbs, among other minor improvements. However, it retained the acetate filters and suffers from the same problems. Modern Photography magazine reviewed it when they tested the D4 enlarger. I scanned the owner's manual and it is available here.
Chromega B Dichroic
The first dichroic model for the B-series. Dichroic filters do not fade or shift color over time, making them much more desirable than acetate. This model also has the modern black box look, rather than the Frankenstein appearance of the original Chromegas.
Super Chromega C Dichroic
The first dichroic model for the C-series enlargers. It's unusual because instead of knobs or drums, it uses horizontal sliders to set the filtration. Unlike the other dichroic heads, it's gray instead of black.
C700 Dichroic
An upgraded replacement for both the B- and C-series dichroic heads; it could be fitted to both series' chasses.
C760 Dichroic
The last of the C-series dichroic heads; it's a minor upgrade of the C700 head.
D5500 CLS
The most advanced of the D-series dichroic heads; it uses computer control and a remote keypad to control the head, rather than mechanical control. Unfortunately, it is no longer in production.
Super Chromega D [E] Dichroic [II]
This long running dichroic head (at least the II model) is still in production after some 35 years. It's a standard knob-controlled mechanical head. The II model offers some minor updates and features.