Welcome to Stovebolt.com, Jmast

It has been reported/suggested that those marking were made on the assembly line.

It could be an assembler #, or an inspector #, or an option # (it does not match any 1949 truck options).

Some people have reported in being yellow/white paint or crayon?
https://www.stovebolt.com/ubbthread...l_markings_for_1949_310.html#Post1084623
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1072031

There was a post a while ago that showed a variety of firewall markings.

Can you post a link to a photo of the marking?