From all B-25 manufactured only this a/c has a s/n ending on 4925.
Could you find out which of the 3 B-25 you mentioned before is this particular one (based on the names of the planes).
P.S. You might have seen this picture as well - a modified B-25G-5 (new designation G-12) from the 42nd BG. This is the exact same modification as the one of your granddad.
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