Hi Mike,
Could you specify the S/N or Date of the GPW in question? Note that
early GPWs (Before mid-late '42) did not come from the factory with a
BO drive light, there was a retrofit kit that was available which
included the "U" shaped guard you have likely seen. This guard was not
"F" marked. Also, there are 3 types of filterette that were used.
-BK