a 9' bed was available in the 40's, possibly earlier I'm not sure.
I know AD trucks have 9' beds on 1 tons and have 4 stake pockets, mine has one of those beds.
I've seen them on late 60's trucks but don't know any more or other years or how many pockets.

I can measure mine sometime if you like, don't know if they're evenly spaced or not?. Honestly though it would make a whole lot more sense if you just spaced yours evenly for your bed length.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-