Looks like a 1H to me.
Before you get too excited about a 5 speed, be aware you’ll probably need the 2-piece later and stronger transfer case as well. AFAIK nobody has succeeded in attaching an older 1 piece TC to an H55F.
My solution on my OJ50 is to stick with the wide ratio H41 transmission already there (has sufficiently low 1st, 4.925:1, to easily start with taller tires), and use an Orion TC with 4:1 low range. Switched from 7.50-16 to 8.25R16 (31.7” up to 33.9”). An effective 6.5% reduction in rpms. If you want a vehicle “that will cruise at 80-85mph on the Interstate”, forget it, get rid of it. Cruising at 60-65 safely is as far as one can go with older Landcruisers. And Unimogs (I also have a U500 camper).