Hmm - leather strap is not ringing a bell with me, and the first pit job (underneath) in my area was the spare tire install. I'm wondering if it was just to help protect the tire from rubbing on the frame or the carrier.
Chevy trucks did come with Uniroyal brand tires. Truth be told, there was motivation to supply trucks with the cheapest tire possible, because any savings on one tire was multiplied by 5 (four on the ground + the spare, if it got one; spare was optional).
Here's the link to the info kit for a '78 Chevy pickup from the GM Heritage Center archive. This indicates a G78-15B tire as standard (bias ply).
http://www.gmheritagecenter.com/gm-h...olet-Truck.pdf
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