frame numbers weren't in general use until after 1955, but I think some GMCs had numbers stamped on the frame rail at the front of the pass side - if not there, nowhere .... also note that GMC didn't change up numbers yearly, very likely that 51 continued a series numbering from 47 .... look around here

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics