Are you looking to do an actual numbers matching build or functionality?
If you look into these crankshaft pulleys, you'll find that the 6312 is the actual pulley. The alphanumeric numbers before are for what it came on, a car in 69 or maybe a 79 truck [if I remember correctly]:
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This is my post that I was going to use on my build, but I didn't need the PS groove, so I didn't want it hanging out there, looking like a belt was missing. If your interested, let me know and I'll knock the price down.