Question about markings on P60 lamp assemblies

[ConfederateScott]

I was just noticing my extra P60 lamp assemblies rowed up on my shelf and discovered the different markings on them. Each one of them has a different letter/number combination in black ink on the back of the reflector. The markings are: QD-1, UD-4, WM-2, WM-3, WJ-2 and also I have a P90 stamped RF-4. What do they mean?

 

[Size15's]

Those are assembly numbers from the manufacturing process.