This is our high quality reproduction of the old lens used on many 1950s trailers. It is a dead ringer for the original used on Airstreams from 1954 to 1958, Aljoa in 1955 and on Boles Aero from at least 1953 through the late '50s.
These fit inside the No.9 or No. 8 metal trim ring (available separately). The only difference between the No. 8 and No. 9 fixtures is that the No. 8 fixtures had a single #1157 bulb and the No. 9 fixtures had one #1157 bulb and one #67 bulb.
Bargman also used No.11 on this lens (and we don't know why), so go by look and size. Lens is 4-1/8" in diameter. Held in place with 4 machine screws. Opposing screw mounting holes are 3.5" apart.