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.
To the best of my knowledge, the XL designation refers to an extended-length column.
I have a few of Instruction Manuals and brochures available for download. 
Quick Comparison
| Model | Mfg | Col. Type | Col. Height | Lens Focus | Focus Method | Baseboard | Price | Notes |
|---|---|---|---|---|---|---|---|---|
| E3 | 1948-1953 | Tri-Rail | Unknown | Cone | Autofocus | Unknown | First to use autofocus. Used a dedicated cold-light head | |
| E4 | 1953-1955 | Tri-Rail | Unknown | Cone | Autofocus | Unknown | Same as the E3 but it used the Omega Light E improved cold-light head | |
| E5 | 1955-1973 | Dual-Rail | Unknown | Cone | Autofocus | 22"x24" | A scaled up version of the D3 | |
| E6 | 1964-1973 | Dual-Rail | XL column | Cone | Manual | Unknown | Manual focus, extended length version of the E5 | |
| Chromega E | 1972-onward | Dual-Rail | Unknown | Bellows | Manual | Unknown | The Super Chromega Dichroic II head is the same as found on the D5, but scaled up to cover 5x7 | |
| Super Chromega F | 1970s | Unknown | Unknown | Bellows | Manual | 24 x 32" | $13,800 |
Series Comments
Omega's largest "series" of enlargers (they made an F, but only one model). The D- and E-series are closely intertwined since they handed similar-sized formats; so features that appeared on one typically showed up on the other sooner or later.
Model Comments
Model E3
The first of the series (that I'm aware of). It introduced Omega's autofocus system, whereby the lens focus varied with the column height by means of a wheel and an aluminum track that was matched to the lens. This model used an early cold-light (i.e. flourescent) head that required it to be left running, and a shutter had to be used to regulate exposure.
Model E4
This is later variant of the E3, except that it used the improved Omega Light E cold light head, which did not need to be left on, and did not require a shutter.
Model E5
A scaled up version of the D3.
Model E6
Like the E5, but with manual focus and fitted with an extended column for larger magnification.
Model Chromega E
This looks a lot like the D5XL, both in chassis and its use of the Super Chromega Dichroic II head; except that it's scaled up to cover 5x7.