Jeff - Just a thought after not being able to find much of any info on your GC that came without a bed (would have been identified as "CCAB" on the data plate I think according to the below snip from a
1937 Chevrolet data PDF from the GM Heritage files).
I think the bracket would have been mounted on the end of the frame cross member (left side) similar to the larger 1-1/2 ton flatbed models (if there are any holes there). Otherwise they would have had to use a right angle mounting bracket to bolt it to the frame end side rail holes you show.
As far as finding info on a "factory flatbed" for the 1/2 ton, none of my resources are giving up any info on that, at least yet. Still looking.
