Here are some pictures of a '56 GMC 1-ton that I came across recently.
19.5" tubless wheel mounted without a pressure plate using 1/2 Marsden lock nuts. Center of wheel closeup Closeup of 1/2" lug nut (Marsden lock nut) These are 16" wheels for later trucks. I'm not sure of the exact years on the rim on the left but the holes are 9/16"+ while the ones on the right are 5/8"+ dia. 19.5" versions of the wheel on the right were found on 3500HD flatbeds and P30 vans (bread trucks, tool vans etc.)
Mid 60's (?) 16" dual and mid 80's 16" dual 16" wheels side by side Mid 80's wheel with tapered lugnuts and pressure plate mounting on 9/16" studs.
Mid 80's up 8-lug mounting Pressure plate and 5/8"+ wheel holes This last picture is the 1-ton hub and drum. The 16" wheels are not even close to slipping over the brake drum.
56 1-ton hub and brake drum. There is a picture of a 2nd series 1-ton GMC on oldGMCtrucks.com. It has the 18" tube tires on it and a pressure plate is clearly visible on the front wheel. This is on a 15,000 mile original truck.
When I bought the 16" wheels with the taper flange plates, there was another wheel available that used the flanged lug nut. The rim was a completely different part number. I bought the rims at 6-Roblees in Seattle and they have on online catalog.
