[C10 - C90 Bill]

SPID is short for Service Parts Identification.

It's a label that tells the info on your truck when it was built. I didn't indicate where it would be on a '74, as my knowledge pretty much stops at '72. But like Bob has indicated check the inside of your Glove Box Door. They can be a little hard to decipher, but look for the info for the rear. If necessary, try to post a picture of it. Hopefully it is still there.