3/4 ton and 1 ton AD trucks have 8 lugs.

A model 3100 is a 1/2 ton truck.

My guess is someone swapped a 3/4 ton rear and did away with the enclosed driveshaft, and gained some slower gears.

Post some pictures.

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-