Yeah, that says it in a nut shell. But if you're like me you'll want some explanation. In my old wheel references the '35/'36 Model 51s are shown to use either 20" x 5" (3.75") or 20" x 6" (4.33") wheels and rims having 5 lug x 8" bolt pattern that was introduced in 1934. The measurements in parentheses are using the new National Wheel and Rim Association (NWRA) rim width measurement standard while the others are using the obsolete NWRA standard. I'll explain below why this matters.
It's important to realize when dealing with these old trucks that wheels could have gotten damaged and replaced over the service life of the vehicle. So what came on the truck may be long gone, and you'll have to deal today with what you've got.
The original wheels were made and sold by the Budd Company, but had outer rims made by either Firestone or Goodyear. In your case it looks like the dominant rim design was called a Firestone "RH" that featured a solid/continuous unbroken locking ring. The "RH"/"RI"/"RIP" rim/ring combos are the most challenging to work on because the ring must be forced past the lip of the rim when mounting. Too aggressive or meat handed treatment will bend or break them, then you're in deep trouble. On the good side, this rim/ring design was a dominant style and was used up into the 1950s. I have a NOS 20" x 5" (new NWRA standard) with an "RH" rim that is date coded 9/53. My point is that you should be able to find an "old guy" service tech that knows how to handle them. If your truck has had wheels replaced, you many find that you have a mix of different rim/ring designs. It is imparative that your service tech keep the ring/rim sets together. Even different widths of the same rim design used differently numbered rings. Mix-matching rim/ring designs can be deadly.
Below I'll link some pictures of the "RH"/"RI"/"RIP" design since it's the most likely you'll have and the most challenging you'll face. I'll also try to link a current OSHA references that show proper service tech handling of multi-part rims.
First is the RH/RI ring. This will be confusing, but of the RH wheels I've pulled apart, even my NOS wheel, they've had RI rings. The catalogs show that the RH was used with the Budd style wheel, while the RI was a demountable rim like used on Dayton applications. In this one instance the rings appear to have been interchanged on like width rims. The RIP, however, was used on smaller diameter wheels as best I can tell. I have a set of these in a 16" size.
Now an old thread from FTE where I linked in two OSHA web sites. The first covers the service procedures and the second covers the risks of mix-matching rim/ring designs. Stu
http://www.ford-trucks.com/forums/686250-multi-part-wheel-safety-osha-documents.html#post5470496