"If you remove the left sideplate there should be a number on the frame"

Hopefully that's true in this instance, but it's not always the case as I've seen a fair number of examples with no stamp beneath the lock plate; the most recent being a Monogram Grade listed in records as a Monogram but having no grade identifiers anywhere on the gun. Yours certainly appears to be a very fine conditioned example Grade 3 gun; and perhaps the best way to authenticate its grade would be to request a research letter. That said, I'm also aware that some serial number records in the 40XXX range are missing; hopefully this wouldn't be one of those. Thanks for sharing your beautiful Smith.