I have not seen a reinforcing plate like you describe on an AD truck frame. Have seen something similar on earlier trucks though.

My guess is not factory, if it were they would have used rivets like on the rest of the frame.

Even if it is old and not factory it could look nice, there were skilled fabricators back then just as there are now.

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-