I guess someone might have cut a 5" but I don't know who in the world would, they're not exactly common either! The fact remains the best answer you can probably get is only with 99% certainty w/o a letter. You might as well get one, if it confirms it can only enhance the value of the gun as well as put your mind at ease. If the letter comes back and does not confirm an original 4", you're no worse off.

For the record, I'd vote original 4".

My pre 26 has a centered barrel marking on the right side, I couldn't tell by looking until I measured it at 4 1/4". But perfect workmanship. And I only had to pay less than 1/2 of an original even if I could find one.

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Jim
S&WCA #819