I can't tell you the specific bulb.
Let me complicate your life with some
allied issues. Fitst, in 1989 the USCG I believe was still using "visible at 1/2/3 miles" with no operational definition of "visible." Since then they have come up with standards that an
engineer can follow, and the fittings now come with the bulb specified. It's moulded into the plastic by bulb number. You can increase the brightness of your abchor light big time while still saving watt hours, but you could be on thin ice in a
legal dispute concerning your light, as in, Joe SixPack didn't see it and T-boned your
boat. Second, I'm of the not too humble opinion that a
single anchor light is not enough. It gets
lost among stars or city lights, and is hard to triangulate on in any case. I hope that you will consider adding additional lights to define your
boat while at anchor. We use outdoor
rope light LEDs to define the wheelhouse roof.
Good luck. One approach would be to buy a modern fixture complete with bulb, and send your friend up the mast with the tools to completely replace the fixture.
Manatees don't climb masts.