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.

The GE M150 100w and 175w MVs in Rhode Island were definitely 1980s. So FCO in MV existed then. Caltrans in the LA area also began using FCO replacements for MV freeway lighting in the late 80s. As late as 1987 they were still installing new MVs with glareshields.

 Just looked at a 1977 GE catalog which listed both the M-250 & M-400 as available in MV .

The first generation M-400A FCO (sealed optics) I know for sure was offered in MV.

I read their old early 80s and late 70s catalogs and it is listed but not their mid to late 80s ones.

It seems that in the mid to late 80s they offered them in only HPS but in the 1990s they brought the MH and MV ones back. I recall seeing an MV M250 FCO once. Either GE lazily only applied HPS in their spec sheets or they stopped making them for a bit.

They never stopped making MV FCOs until the 2008 MV ballast ban. Just not as common as HPS.

Not sure if this is at all helpful, but here are a couple of MV M-400A2 FCOs, likely from the early 90s-late 80s

https://maps.app.goo.gl/tXcqDhsYnRFdjH1HA

https://maps.app.goo.gl/a3Bq8Z8QKk6AZMYo9

So, GE just got lazy and didn't put them in their spec sheets, but they were in fact made in MV. Interesting. I notice their M250R1 spec sheet one of them I read said “metal halide and mercury vapor options available too”, so they were indeed made. I’ve only personally seen drop lens M400A2’s in MV.

Not sure if this is at all helpful, but here are a couple of MV M-400A2 FCOs, likely from the early 90s-late 80s

https://maps.app.goo.gl/tXcqDhsYnRFdjH1HA

https://maps.app.goo.gl/a3Bq8Z8QKk6AZMYo9

Those are definitely MV. There are some posts on here about metal halide versions of the M400A2 FCO as well. So, GE did make MV and MH versions of the FCO M400A2. I know MV FCO’s were common in some places, they were widely used in Maryland and Charleston also used some FCO MV’s. Some places in Rhode Island I heard also used them. I heard Boston had MV FCO cobra heads as well.

I recall seeing several MV FCO’s in my life but most commonly I saw M250R2’s, 115’s, and M250A2’s when I saw an MV FCO, I’ve seen FCO and even sag lens 125’s and M4’s that were mercury vapor. But have not yet seen an MV M400A2 FCO in mercury vapor. Even though I’ve seen drop lens M400R2’s and M400A2’s in MV not too rarely when I traveled. I recall seeing an older FCO M250 that was Mercury before as well. According to Silverliner, some parts of California used MV FCO’s. Kind of interesting.

At this point, the rarest Mercury vapor fixture might be newer flood lights, followed by reproduction acorns or teardrop fixtures, rather than FCO cobras. Charleston used MV flood lights though. Charleston used pretty much everything in MV, even decorative acorns!

Cooper FCO fixtures seem to be the rarest to find in MV out of all of them. OVZ’s and OVX’s aren’t too uncommon to find in MV in certain places, but every other Cooper cobra head seems very rare in mercury vapor. I don’t even know if Cooper made Mercury OVH’s or not, I’d assume they did but I can’t find any spec sheet listing mercury vapor as an option.

Fixtures in Charleston seem to always have NEMA tags. Even acorns have them. Duke Energy seems to put NEMA tags on everything.

So, companies did make FCO’s in MV, they just didn’t specify it for a while. At this point, pretty much no fixture is rare in mercury vapor in all places, except for like reproduction teardrops or AEL 153’s. People who say “the AEL 113 is rare in MV” have probably never left a specific part of their state or live in states that lack MV’s or any newer MV fixture. When the AEL 113 is probably one of the most prevalent MV cobra heads in the U.S. when it comes to modern fixtures, and so is the M250R2.