I have a blackout headlight on my M38.
The front face of the (sealed beam) blackout bulb is embossed 4-1/2 (inches).
I didn't take the retaining ring off, but it measures pretty close to 4-1/2 inches across.
A quick google search turned up lots of 4-1/2" sealed beam lights.
Wagner 4411 is 12V and 35W available at most any auto parts store for about $15
Sylvania H4411 is halogen 35W and 12V for about same price.
I also see some in 6V if that's what you need?
Looks like 4511 is the part number for 6V.
Most military blackout headlights are 24V.
Looks like 4578 is the part number for 24V.
Hope this helps.
Good luck!