Mystery firearm part - What is this?

[gunsrfun1]

I was going through some boxes and found a small brown cardboard box with a foam insert, and opened it up. I found this part inside. (See attached Word doc.)
Pretty sure a gun came shipped in this box, but I have no clue which gun, since it's not labeled. And it may not even be from one of my guns; could just be a box I picked up somewhere.
All my guns function properly, as far as I know.
Any idea what this is? Maybe it's not even a gun part, but I thought I would start on that assumption.
Thanks

 

[earplug]

S&W . It is the part that fits into the cylinder notches, operated by the front of the trigger. Can't recall if its called a lock

 

[Obturation]

Looks like some kind of locking block, only a guess- I don't actually know at all

 

[gunsrfun1]

Thanks both of you.
Earplug, interesting that you mention that. I did recently buy a used S&W Model 60-14, but I picked it up at my FFL in the original blue plastic box, not in this box. It's in like-new condition and appears to be fully functional. Perfect timing and lockup, etc. I can see the firing pin activate when I pull the trigger and the transfer bar drops. (I haven't actually shot it yet.) So I doubt it is from that.
I have two other S&W revolvers (66 and 686), but both are fully functional and haven't come near the cardboard box ever.
Wonder if it's just a random part that a previous owner may have left in the box. But I have no recollection of where I got that box! (And it may not have contained a firearm anyway.)
I'll hold onto the part and see if any of my guns start misbehaving.
Thanks again

 

[BBBBill]

That is the cylinder stop. It pops up through the bottom of the frame window to engage the slots in the cylinder. It is a fitted part, that is it usually requires precise fitting to the individual gun. There are numerous surfaces on the stop that are addressed in the fitting process to get perfect function and lockup. They used to be available as an oversized part along with oversized hands (the part that turns the cylinder) to repair guns with frames that were worn oversize or possibly machined a little out of tolerance. Great for super tuning a match gun, too. I think Ron Power may make hands for the trade now, but I don't know of any oversized stops.

One thing for sure, your gun will not function correctly without a stop in place. I think you just have an extra part.

 

[gunsrfun1]

Thanks BBBB, that sure is strange. I did have my S&W 38 repaired by S&W about 5 years ago, after it went out of time. (Cylinder would not lock up.) It's possible that's the part they returned with it when they sent it back, but I have no idea if that is even the box it came back in. Could be a coincidence, but that is the only thing I can remotely think of.
In any case, sounds like I have an extra part, as you say, so I'm not going to worry about it. Who knows, maybe it will come to me in the middle of the night sometime.
Thanks again for the clarification.