Very interesting indeed!
500 black was an option on series 40-90 only in 73, not light duty. So, I'd have expected you'd have a Special Paint code on the SPID rather than the color RPO. Kinda breaks from precedent. As a data point, very similar situation for me with a 76 I had - EXCEPT it had the Special Paint code. It was 88 orange, and in 76 that was only available on series 40-90, so the SPID made sense.
Anyway, you've got an original black 73 and that's pretty awesome!