Hi guys,
While I was searching for nice walnut grips, I was surprised to find this gun with a front sight looking like mine (But I think it was re barreled since the serial number seem to be over other numbers) and the same hole in the trigger guard
S&W Smith 1905 3rd Change M&P Target Model 38 - Revolvers at GunBroker.com : 797052195
Yes, the front sight looks like yours, but is forged as part of the barrel (not pinned in place). I'm quite certain this barrel is from a fixed sight gun.
Ralph Tremaine