Looking at several original 66-67 pumps I see three types of pulley retaining bolts; RBW, RBW with the grade marks, and plain head. A smog pump I have handy has a date of March, 1966 and has plain head bolts. I can take a look at some others to try to find a pattern as to when which bolts were used. All fall under 20311-S, and require a 34806-S lockwasher.
As far as the 42973-S castle-head self locking bolt, it appears to me that that was a pre-production number for 1966. If you look closely in Osborne's 66 engine manual, there are basically two sets of pulley/belt arrangement pages for with emissions; all the illustrations showing the 42973-S bolt are dated April 1, 1965 whereas the illustrations showing the 20311-S bolt are dated November 12, 1965. I don't recall ever seeing any early 66 pumps with the 42973-S bolts, but next time i go to my warehouse i can see if i have any.
As an aside, the 65-72 Ford MPC as well as my 60-68 MPC show the 20311-S and 34806-S combo used to retain the clutch spring bracket to the pedal support for 65-66. I think I've usually seen SEM bolts used there, but anything's possible.
garrett