What I’ve noticed is that when FCO fixtures first came into the market in the 1970s and 80s, there were no mercury vapor or metal halide versions, only HPS. It seems that Mercury vapor FCO’s weren’t being made until the 90s. It seems to be very rare to find a vintage MV cobra that’s full cutoff, while newer ones are actually relatively common in certain cities (notably Charlotte, NC and certain parts of Maryland). Some parts of Rhode Island also used them. I’ve seen FCO 115’s, M250R2’s, M250A2’s, and even newer M400’s and 125’s in FCO with mercury vapor lamps, but never have I found a vintage FCO M400A or a model 13 in MV. Both AEL FCO’s and GE FCO’s only listed HPS in their spec sheets.

This is very interesting since usually HPS fixtures were installed later than MV ones, but exceptions seem to apply. Most older MV fixtures are the traditional drop lens style. Newer MV installations can be FCO or drop lens similar to the HPS ones.

Same goes for MH cobra heads (as a whole), MH cobra heads didn't become somewhat common until the 1990s. Metal halide cobra heads are actually quite common in South Carolina, certain parts of Texas, and in Cincinnati, OH. Older FCO cobra heads in MV seem rare all around. I’m wondering if the early Cooper fixtures like the OVM or OVS were made with FCO and MV. I can’t find any spec sheets for those so I don’t know what lamp types were available for those.

We had FCO lanterns in the 1950s with mercury and LPS in the UK. They were usually standard lanterns with 'aeroscreen' shields to prevent upward light, or modified bowls made of metal with just a clear panel underneath. 1960s and 70s saw purpose built FCO lanterns for LPS.
But FCO back then had a very specialised use. They were only really used by railway lines or airports, so they were very rarely seen. It was only when HPS lanterns came out in the 80s that they started having FCO optics as a common option, and the last generation of discharge lanterns in the early 2000s and LEDs after are when FCO became standard.