Price (new): $14.50
Measure type: Comparometer
Battery type: two AA (I believe)
This model D was a departure from the previous meters. Whereas the Models A, B and C all had the same case and layout, and differed mainly by their calculator dials, this Model D has three major changes.
First is the cosmetics: it's slimmer and a horizontal rectangle, similar to the Weston 650; which probably made it a little easier to hold.
Second is the movement of the comparison light from being next to the viewing window to being inside the window itself, which probably makes it a little easier to do the comparison. The SEI Photometer does it that way.
The big difference is the addition of a compensation control knob. All the Phaostrons (in fact, all comparometers) run on batteries and they light a little lamp. As the batteries weaken with use and the voltage drops, the bulb gets dimmer. On the older Phaostrons, you just have to change batteries every 4 months or 180 meter readings (yeah, you'll keep track). On this one there's a battery check, so you can make adjustments over time to compensate for the battery voltage. Once you can't compensate anymore, it's time for new batteries. A very nice feature to have.
This meter came just before the US entered World World II, and I believe its rarity is due to that; there weren't many made before Phaostron switched over to war production (they made a variety of electrical equipment). After the war, Phaostron did not resume production.