I finally pulled the trans hump off of the cab to get a better look, and the trans does indeed have the specific top cover that has the provision for a handbrake. Someone decided they did not need it anymore or it broke and they just left it as is.
After scraping about a pound of muck off and spraying it down with cleaner I was able to read the following numbers, 3830574, and A2364. What I think I have here is an SM420 that was cast on January 23, 1964. Perhaps someone can confirm.
So this is a conundrum, kind of, but here is what I know or am pretty sure of:
- This truck does indeed have the J76 Prop Shaft Park Brake option on the SPID, ie. from factory. If it wasn't right there on the SPID I wouldn't have believed it and thought maybe this transmission was an early swap.
- This type of J76 parking brake was somewhat commonly offered on light trucks as late as 1966, but I have seen only two or three 1967-1972 C10 or C20's that have it after searching 20 years of discussion in this forum.
- ALL of the few that I have found were made in Oshawa, Ontario Canada, including mine.
- Despite being a 1968, it has a transmission that to the best of my knowledge was discontinued in 1967. I have never seen it's replacement, the SM465, with a prop shaft brake attached to it.
- Even more odd, the transmission that is in it is stamped 1964. I have absolutely no reason to believe that this transmission has ever been out of the truck: the truck is in horrible, original condition.
- Assuming that all of the above is true I am going to guess that the Oshawa plant was using up old transmissions for a few years. My 71 year old Chevy mechanic friend says that this is plausible, he started his first job around this time and (he thinks) he remembers this happening at the Oshawa plant.
I don't know if the trans hump is original... looking at it I wonder if it was originally blue, I see some red underneath: regardless, it has been removed and reinstalled at some point, maybe because it was replaced with one that eliminated the beside-the-shifter handbrake slot that was common up until 1966. Or, maybe it is original and the brake was operated from under the dash like all 67-68s were, with a special cable and different routing... I am doubtful of this, though, as the trans clearly has a spindle that the handbrake rotates on, and it does show wear from use. Regardless, I am thinking of installing a handbrake next to the shifter as I have seen all pics of this set up do, and make a spot in the trans hump for it to go through.
I feel like I am solving a mystery of sorts, it's too bad that this extremely rare option doesn't make the truck more valuable, just more difficult to restore. Feel free to chime in, this all started because of a "rare option SPID" thread and I got curious about what the heck this was.