My 56 has the numbers stamped in that location. It is an LA built truck. I have seen several others also. You may have to remove 50 years of paint,crud, & corosion. It's not stamped very deep so it's possible the elements took their toll.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine