My last answer must be only appearing on my PC as you haven't commented seeing it ...............
I have known RPO to mean Regular Production Option for many years of reading Shop Manuals.
.... they put this in (spasmodically), wherever there have been REVISIONS (Improvements) to alert readers as to why their car may not have an item looking, or being, exactly as seen, or described in the particular Shop Manual, revised or original, as you may be happening to be reading at the time.
Another way of telling you not to panic if you see something that doesn't look original, as it may have been changed during production or on a "recall"
"GOOD BETTER BEST: NEVER LET IT REST: UNTIL YOUR GOOD IS BETTER, AND YOUR BETTER, BEST"
( Furphy WW1 HORSE-DRAWN water-tank end-plates)