Bargman 99 vs 98 question

If my feeble memory serves me at all (.... doubtful at best! ...) I believe you are correct, but I am no expert.
There really wouldn't be any other real reason for the designations, or for the two-bulb configuration, for that matter.
Pathfinder lights are also designated in much the same way. There is a 575 and a 576, one is a single-bulb arrangement and the other is a two-bulb setup. On my 1970, the two-bulb is wired so that the second bulb is in the clear cutout to act as license illumination and is wired to the running lights.

I have, however, seen the two-bulb lights wired with the #67 bulb illuminated with the brake/signal circuit, and acting for those functions, while a single filament bulb had been used in other socket and wired with the running light circuit and acting only as tail markers. In these cases the two-bulb light was used on both sides. In addition, the license was normally centered in the rear with a separate light from the tails.

You might try expanding your search to include the Pathfinder 575/576, and perhaps the Reflect-o-Lite 110/150 just to give you a broader search base.

I have my bargman taillights up and running, much better than the square boat trailer lights the PO installed. This gets me back to the original 7 way socket, which I gather buys me 12v charging of the trailer battery and electric breaks, but I guess I will figure that out later.

Untitled by Tom Myers, on Flickr

I did overlook the need for a clear lamp for the license plate though. I think the plate was in the window when I bought the trailer, and now not completely sure where the original plate went. Searching, I see a few Yellowstone's with the plate under the left taillight so guessing that is original.

plate2 by Tom Myers, on Flickr

I do like this look a bit more though, a bracket attached to the bumper, even if not original.

plate by Tom Myers, on Flickr

Anyway, big question that was not answered on this thread and I cannot find elsewhere, has anyone found the reproduction bargmans or similar size lens having the clear cutaway in the bottom portion? With that, the running lights would shine clear on the license plate, and I can avoid the additional light fixture exclusively for the plate. VTS does not have them and did not know where to point me.

Or, has anyone tried to fabricate a red lens, replacing a portion of the bottom of the lens with clear plastic, or maybe there is some other material I could use? The lens is pretty brittle, but thinking if I take my time with a dremel (not a saws-all, ha) I could cut away a section, but then need to figure out what clear material would be good to insert? As the clear insert would be protected by the fixture itself, I am thinking it might not need to be water tight either? Any pointers appreciated!