Is my gun illegitimate?

[B. Dudley]

The style of buttplate on the gun (if it still has one) will help confirm original gauge.

0 frame 16g guns used a larger buttplate than 0 frame 20g guns. So if your plate is the larger "1" style, then it was originally a 16g.

[David Penland]

There are no repair codes. The serial number matches the gun. Will try and post a pic of butt plate.