Smith revolver question...

On the left side of the frame, in the lower rear corner is a small lump that l would call a cylinder stop. The cylinder rear face touches it when cylinder is out. 

What is the purpose of it and can I shave it back just a tad?

 Reason is, l have a new short cylinder in 45 colt that ALMOST fits my JM 625. There may be other reasons that it interferes such as a long gas ring but l haven't measured it all yet.

Like they say "Think through it, before you do it "

No, l haven't fitted a Smith cylinder before. Thanks

That "lump" (called frame lug) stops the cylinder from falling out when you open the cylinder and hit the ejector rod.  I had an ill fitting "lump" as you call it on my S&W M-60 that I carried as an off duty gun.  If I opened the cylinder and hit the ejector rod the cylinder would eject from the gun and hit the floor. And no, the cylinder wasn't emptied, the cartridges remained in the cylinder when it hit the floor.
They are replaceable so if you feel like experimenting go ahead and save it back a tad but be warned that if you install the original cylinder it just may not stop the cylinder when you hit the ejector rod.

Virgil

Thanks! Makes sense now.

How are the lumps replaced? Mine looks integral to the frame.

The frame lug is a press fit and then staked in place.  You can remove them by taking off the side plate and punching out the old one while on a block with a hole in it to let the old lug pass through.    When installing the new one a dab of Loctite and tapping the new one in AFTER cleaning out the hole with acetone works well, then stake the new one slightly from the inside of the frame so it doesn't twist. Your 625 is SS so blending the new lug to match the old one should be easy enough, it's the blued ones and trying to get them to match the old bluing that's a challenge.

Virgil

Dual cylinder 25-2. 45 acp, 45 Colt.
Each with a fitted hand.

I took a 25-3 cylinder and fitted it to my 25-2. The -3 is the short 45 Colt. I didn't want to modify the frame on the 25-2 so I modified the cylinder to slide past the notch. It was an experiment and I got the cylinder cheap so I didn't go all the way around. Just enough to make it work.

Re-examines my 25-2 and it does look like the cylinder frame lug was madified