If that "CAB No." tag was attached to the truck before you removed it, it may have been changed out (many years ago from the looks of it), and stamped with a replacement engine number. Many jurisdictions used engine serial numbers for registration (I know that California did in the '60s), especially before the current 17 digit(?) VIN scheme was developed. That may explain why the number looks like a '46 1/2 ton engine serial number with the DBM prefix.
