['68OrangeSunshine]

My '68 C/10 Stepside came with 5 of them.
I bought it in June 1973.
It's on the SPID. 17.5 x 5.25 rims, 6-lug, 5.5'' spacing. Tabs for hubcaps. OEM tire size was 7.00x17.5. In the late '70s the 7'' tires were obsolete, so I went up to 8.00x17.5 bias 6 ply.
In the '80s I upgraded to radials: 8R17.5LT.
By the Turn of the Century, [2000] the tire shop guys told me my rims were shot, and I needed new ones. They put me up on white spoke 16 x 6 or 7'' and 235/85/R16LT rubber.
I don't know who the Original Owner of my truck was, but like Jerry Moss says, that size was an unusual one preferred by municipalities, or railroads, or construction companies. [Military also had 17.5 jeep tires, but they had that opposing lug tread. No good for highway use.]
Might have been CalTrans. It's a Fremont rig, and I bought it in Pasadena. Build sheet -- before I lost it -- mentioned Felix Chevrolet, in Downtown LA. Color of solid 516 Orange is a clue.
I have four ''shot'' 17.5 rims and two OK spares -- one was original truck equipment, the other is slightly different, a Firestone-made rim, in MOPAR style I'm told. Still 5.25 x 17.5. If I had 2 or 3 more good 17.5 rims I could look for tires, but I bet they're few and far between, and the ones you can get are multo expensivo.

In their day they were a good tall highway rim.