We can speculate and make assumptions based on the information in the 1954 Truck Engineering Features manual (linked-to above).
The 1954 3100 H-M has a significantly different frame set-up compared to standard transmission 3100 trucks. The H-M-equipped 3100s had a set of two bolt-on frame cross-members: one under the rear engine (where the standard cross-member was) and an additional cross-member under the rear of the H-M transmission.
The extra cross-member was needed for the very heavy H-M transmission; and, the two cross-members were bolt-in to allow the transmission to be easily removed from below (with or without the engine).
