I've seen it before. One reason is bad front cab mounts and or rusted away floor at the cab mounts. The column is more or less ridged and the cab is fixed to it, so if mounts aren't doing their part it gets ugly where bolted to the dash.

One of my trucks has pretty bad cracks there but nothing missing, and the floor is perfect with mounts in place. Could be it was on some pretty rough roads and very little give in 2 ton springs? It is also a 48 with one center rear cab mount, this may allow more cab movement than necessary and stress on the dash/column clamp?

•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-