For C/K20 & C30, it's just a registration/certification thing at no cost to customer. For C/K10, it cost you $15.00 to have more crap on your motor
(EDIT - this statement is incorrect, see the "EDIT" below - would love to find out more from a Fremont assembly line guy! - I learn something new every day)
My 72 K20 has the option also. Kind of a weird RPO, i.e. "option" as it apparently wasn't optional! Kinda like that "Canadian Base Equip" thing - if a Canadian, it wasn't exactly optional.
Here's the VIK excerpt below. What I find interesting is that the option was "NOT available on 307 or 402"... So, it's just a 350 thing? (and 6's?? now that's a good question). HOWEVER, I could've sworn the unrestored 72 402's I've seen had a smog pump...
EDIT: actually, as I read it again and dig thru some SPIDs and old pics (and re-read Pete's post), it appears the buyer was just paying to have the emissions test performed on the 350 (and 6's?) as they come out of the factory - i.e. mandatory YF5. Not required for 307/402, and C/K20 & C30. That makes a little more sense I believe - because I don't see the YF5 code on 402-optioned trucks, but I do see it on 350 optioned trucks (all Z-Fremont).
Now that I've restated the obvious...!