I wondered about this too when I first got my truck. I think that they started making the cab bumps in '69 (thats what I have, if you couldn't tell :p ). I believe in '67 and '68, they used the chrome teardrop screw on variety. The lenses aren't really rare, because they didn't just use them on 69-72. They used the same clearance lamps til '87 (for reg cab pickups, '91 for burbs and crew cabs). They were also used on medium trucks in both time periods, but I do not know for sure about heavies. If you need a set, just go to the junkyard. I went and found probably 5 (73-87) C/10's with them, and 10 or more (73-91) C/30's with them. Alot had broken or cracked by the screws, but I found one truck with what looked like 3 brand new lenses, so I nabbed them for my GMC (It had a mismatched set before, 2 of this style and 3 of the hockey puck style, not sure what those are from). Hope this helps. Oh, and yes, my lights are listed on my option sticker, 3UO1AA.
