If the Ser# is changed (on the frame,,and I don't know that it is) to make a No.4 out of a Field for example,,who ever did the 'work' most likely added a grade stamp into the lock inlet on the frame also.
Just the fact that there is one stamped there doesn't mean it's legit.
Just like a factory letter confirming a ser# shipped as a certain grade/configuration. It never says that the gun you read that number off of is THE gun they made & shipped.
As I said before,,looking inside this thing would be very interesting.
But from a few feet away the caution flags should have gone up,,and they did for a lot of folks here looking at pics.
..and how 'bout that pic of the fixed up by hand matted top rib Tom,,hungry today :>)
(Just having a little fun!,,appreciate all the info you and the others that do the real research provide.)