My grandfather, who was a preacher, told me one of the hardest lessons he had to learn in his profession was to accept offered gifts graciously. Maybe the current owner of this gun has other people he could offer the gun to, or maybe he doesn't. In either case, he seems to have offered it to you first. It won't hurt you in any deep way to say yes, and you can assure him that if it doesn't fit into your collection you will find the right person to pass it on to after giving it some TLC.
With that out of the way, it looks like a 1957 gun to me: Non-model marked, four screws, serial number not marked on rear face of cylinder. Those characteristics pretty much limit its year of origin, though it might not have shipped until 1958 if it got stuck on a back shelf in inventory. I'm guessing the serial number lies within shouting distance of K300000, but probably not lower than K275000 or higher than K325000.