G2 and awilli,
I don't usually visit the rimfire section of this forum. So thx G2 for the pm. Between the two of you, you have hit on all the probable causes.
awilli, as you posted in your first post, it seems to be a cyl lock issue. This is because of a slight variation in parts tolerances common in all guns.
Changing the plungers in the hammers is what I also would have tried first, but as you measuered, the differences in length are slight.
I believe your cyl lock bolt is on the tall side, and doesn't clear the cyl notch (as you suspected) by the time the pawl (which may also have a top tooth a bit tall) tries to rotate the cyl. You can test the theory; wrap some strips of masking tape around the front of the cyl lock. This will prevent it from rising too high. Make sure the cyl still locks in position with hammer down and gate closed. Or just go straight to #1 below. Now see if the hammer will cock.
If the gun functions properly you have two choices of resolution:
1. Knock the sharp edge and right front corner off the front of the top tooth of the pawl. You can do this with the pawl still in the gun. Take a stroke or two at a time and try the cylinder. This is the easiest and what I would do. The top tooth is not that critical to timing and hard to mess up.
2. Dress down the top surface of the cyl latch maintaining the angle and round shape. The advantage of this solution is that if you take too much off, you can fix it by dressing down the 'shelf' where you put the test tape.
Good luck, you'll get it, these simple adjustments can cause great frustration, but minor issues.
P.S. MY SHORTCUT FOR REMOVING JUST HAMMER and/or PAWL:
Recognize, you do not need to hassle with the PESKY LOADING GATE SPRING, pull the trigger pin, OR trigger/transfer bar JUST to remove the hammer and pawl. Once you remove the grip frame, the hammer pin is the only other part you need to remove. After that, just pull the hammer back and all the way down, then depress the hammer plunger in the base of the hammer with a small tipped screwdriver to clear the trigger extension where the transfer bar connects to it. Let the hammer & pawl fall out. Reinstall the hammer and pawl back in the same way they came out. Push the plunger in by pushing it against the trigger extension just like you did with the tip of the small screwdriver, and the hammer will slip by the trigger extension. Piece o' cake.
If either the trigger or hammer pin are snug fits, after they're removed the first time, it's the opportunity to polish them so they're slip fits to put back in and remove the next time.