The 47's had a smoother looking firewall, and I think in 48 they changed and were the same, or close to the same, for the rest of the AD years.
It would be good if someone had pictures, as I've never seen a 47 side by side with the later model to compare.

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-