My '55 has the number stamped on the frame not once, but twice! It has the one next to the steering box (easily visible), the other is roughly at the center of the cab floor. So far, all TF trucks I've looked at have the stamped number. I can't/won't say for GMC, as I really haven't looked at many of those.
BTW, mine came from Baltimore, and was originally sold in N. Carolina.
