[Mr70]

Hi P.J.
#3900434 AB is a water pump pulley originally used on the line for 1967-68 Camaro-Corvette w/327cid w/o A/C.
And all 1967-68 Impala series w/SB w/o A/C. & 6cyl. w/P/S.
And 1967-68 series 10-30 & Dual Duty Trucks w/SB w/o A/C.
And Recommended replacement for earlier:
1964-65 All w/SB.
1966 BB w/62 Amp Alt.

Could you mean #3770245 CD?
That's a Water pump pulley originally used on the line on the 1967-68 Z/28 Camaro.
And 1967-68 Series 10-30 Truck w/SB & 6cyl. w/P/S & 62 amp. Alt.,but not when A/C equipped.