{Collins} Filter cover on RE 75S-3C

Does anyone know what variants were made for the 75S-3 B and C? I have three different ones, one only has the top painted gray, one has no paint and a foam insert, Collins PN 13499 553 4353 001 B, no dashes between the numbers/letter.. All in a single line..is this a late version?
Another is painted all gray, same part number but no letter B, 13499 is on the top line 
553-4353-001 is on the bottom line, the last one is only painted gray on the top, the ears that hold the screws are not painted and it has the numbers stamped on it on green, no letter B.
Just curious as the shiny unpainted came in a late production RE 75S-3B.
Thanks for the bandwidth!.

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