Every one with the chrome side trim in the pics on the page in that link is a 5680 series Elcamino. So maybe the 64 "Custom Elcamino" bench got the chrome ?
My car is a 5415 series 2 door wagon & I'm positive the chrome seat spears are original to the car. The seat still had the original upholstery, albeit in horrible shape, but it was the 300 series cloth/vinyl combo that isn't reproduced, so there's no doubt it was factory installed.
My wagon was kind of strange build from the factory...there's a few oddities that were factory installed that were "mistakes" by the line workers, so maybe this is another one. The car came with a Malibu wagon tailgate emblem which is about 15" wider than the 300 emblem. Both emblems use the same plastic insert, but the bigger one was for the Malibu trim. I'm certain the gate is original the car, because it was hit in the rear & there was some decent mud work done at least 30-40 years ago where they drilled a bunch of holes to pull out the dent. What was still in place was the correct factory medium fawn interior paint color on the inner panel of the tailgate, but the original Desert Beige body color was still on the sides of the inner tailgate where it wraps around to the outer skin. If the gate was replaced, it wouldn't have had the bondo & the holes...nor would they take the time to mask the paint breaks so nicely if they did such hack bodywork.
Also, back to my seat covers, they were a bit of a mystery until I contacted Doug @SMS
. The vinyl portion of the covers had the correct Chevelle 300 triangular heat stamped embossings, but the cloth part had the completely wrong pattern. When I sent pics to Doug, he confirmed they were incorrect for a Chevelle 300, but correct for a Corvair.....& of course he had the NOS material for them in stock. I didn't wind up choosing that pattern when I ordered my seat covers, but it was nice to know it was an original GM fabric that Fisher Trim installed by mistake.
So after all this rambling, I guess I'm trying to say I have no idea why the seats in my car came with that chrome in my lowly 300 series wagon....but I will be reinstalling them....if SMS ever gets me the covers. I digress.