Interesting - mine threaded one is original to the rig, and it's a 8/75 build date.Radiator Pipe part number 16577-61040 is for 1/75 thru 7/80. If yours has a threaded fitting, it is probably part number 16577-60100 for 3/69 thru 1/75 FJ40's. I would think they are interchangeable, maybe?
Those '76 model year but '75 build year can be odd ducks, though. It's a Sep date too, so one of the early early '76s.
Edit: and I'd agree, they seem to be the same form factor.