Originally Posted by anyoldiron
...What came before the SM420?
... both my boxes have the pull up lever for reverse.

That answers the question, having the little thumb lever for reverse indicates you don't have an SM420, but rather you do have the non syncro predecessor.

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-