Cyl latch spring.
But you should always check the latch to cyl notch "fit" first.
Cyl Throw By (over travel):
A new cyl stop may take more fitting work than just "tuning up" your original stop, including to the front cyl stop shelf (see photo below which is a S&W but the principle is the same).
1.First troubleshoot:
The 1st thing you want to know is if the cyl stop is reaching the bottom of the notch and if it's centered in the notch.
"Paint" the bottom of the cyl notches with a black Sharpie felt pen. Cycle the cyl around several times by dry firing in single or double action. Look for a "footprint" of the cyl stop in the bottom of the notch; it'll be a bright spot in the ink.
If no footprint, the stop needs raising, see STEP 2 below.
If there is a foot print, is it a square 'footprint' in the center bottom of each notch. If not see STEP 1.
STEP 1] If the footprint is off center, the top of the cyl stop is not shaped right and does not allow deepest and full engagement in the notches. Stone the top surface to reshape slightly so the high point of the stop leaves a footprint in the ink at the bottom of the notch. A Dremel tool with polishing wheel is good for this if you have some skill with one. Either can be done w/o disassembling the gun.
If the cyl line (cyl stop track) is off center in the approach ramp (notch lead), that's an indication the stop is not shaped properly.
Once you get the footprint at center in the bottom of the notch, check for over travel. If none, you fixed it.
STEP 2] If no footprint in bottom of notch and/or cyl still free spins do this next:
Remove cyl stop from the gun and stone a little off the surface in front of the actual stop lobe. This will allow higher protrusion of the lobe thru the frame opening. Stone until it leaves a footprint. See post #3 here:
http://smith-wessonforum.com/s-w-smithing/541591-model-36-cylinder-can-turned-hand-when-closed.html#post139822921
Photo by armorer951
And then do STEP 1. Once there's a footprint but not in the center of the notch, the stop can not engage the full depth of the notch. And the front-to-back curve of the stop lobe needs re-shaping of the curve to ctr it. This takes very little stoning to change the curve and 'move' the footprint to the ctr.
If there's a round footprint, the stop lobe top surface has a hump from left side to right side and needs flattening with a stone.
Be sure to leave both of the stop lobe edges square, especially the left edge to keep from jumping the notch.
If the side edges of the cyl notches are burred up, 'upset' them back to flush with cyl surface using a flat tip punch that's polished on the tip; use gentle tapping with a very small hammer.
Recheck that the footprint is reaching the bottom of the notch and looks square.
That will do it. Any questions, please just ask.