papi,

I have never seen production numbers for 1955-1st series Chevrolet trucks. I'd guess the maximum number to be no more than 75,000 for all sizes. This guess is based on the assumption of 55-1st selling through March 1955, and the sales being no more than 25% of the 1954 total-production (which was less than the 1953 production).

You are correct about that 3100-series 1955-1st truck being the 7000th truck (all series/models) out of Atlanta for that year's models.