Background is electronic engineer.
The question from Main sail what legal led light to use in old fixtures?
When a Navigation light and fixture gets certified you cannot alter from that unless the fixture is certified with that LED in that fixture. Dr LED makes many LEDs for many uses they are the only one that has taken one of their LEDs and installed in I thing an Aqua Signal fixture and had it certified that way. If any thing gets changed in a certified light it must be recertified with that change, that's the reason for certification.
"Which aftermarket LED emitters/bulbs are LEGAL, and USCG/COLREG's certified for use in older fixtures??"
"The question from Main sail what legal led light to use in old fixtures?"
For a recreational vessel, any light is legal to use in any fixture. BUT no light is legally USCG accepted unless that light has been tested and certified to USCG specifications IN THAT FIXTURE.
IOW, you can't buy an LED "bulb" assembly and stick it into anything, unless it is certified legal for use in that fixture. At least one maker has come out with generic replacements that have supposedly been tested and certified for use in a number of the most popular fixtures--but that's still based on submitting it for testing multiple times, with multiple "listed" fixtures, AFAIK.
I haven't checked on this in the past year or two but expect it is still "the law".