Need help with trans ID

Hey all, This is to be the first post of many, I'm sure. I just bought a '71 Duster with a 318, and the guy said it had a 904 behind it, but my numbers are starting to seem a little off. Can anyone tell me what a PK4431428 9705 is. I'm looking to do a rebuild on it, and would like any information year, type, what it originally came from. Thanks!