This is just a heads up.  Cimarron has not sold Chinese '87 shotguns before, just some of the Chiappa offerings.  But they have had some on order for a long time, and I noticed they have received them.  They are coming from the company that is making the 1878 coach guns, though I couldn't say if it is the same factory.  It is possible that these are really crappy '87's like Century was importing.  I really don't know.  The thing that might interest some folks is that they have them in both 20" and 30" barrels.  I don't know that any other Chinese '87's have had the long barrel.  So for some of y'all that like 'em long, just maybe...

Sorry, I don't know price or anything else about them and not on their website.  If you search around you can still find an image of the Chiappa.  I just saw them on the inventory list.

Well, that busts half my theory, but I still think we're onto something.  I don't think your rifle needs a "break-in" to fix this.  You said that when you move the cartridge back about 1/16th of an inch, that frees everything up.  This still suggests that the bullet nose on that last round still catches occasionally on the magazine tube opening.  If you're agreeable to breaking the rifle down again, I suggest you give the top of the breech-end entrance to the magazine tube a look-see.  I'd be willing to bet that edge is sharper than it should be, and this really isn't a difficult fix.  Are you using lead bullets?  If so, try cycling some rounds through again until you find another last round that sticks, then put enough pressure on the lever to leave a mark on the bullet nose (if that's, indeed what's catching).  Once you free it, just raise the elevator all the way and look closely at the bullet nose, and I'll bet you see a mark there. 

Remember, with no rounds in the magazine pushing on that last bullet, it is free to rattle forwards and backwards however far the cartridge is shorter than the window.  It is also not lying perfectly horizontally, because the rim will cause a downward attitude of the nose, canting the meplat a bit.  Have you tried levering with the muzzle pointed up past 45 degrees?  How about down past 45 degrees?  I'd be interested to know if the condition presents itself with either or both.

If it tends to only happen when the cartridge has slid to the front of the window, you can cure this by working on that magazine entrance.  It really isn't too hard to bevel and polish that top edge. 

It strikes on the rim of the cartridge and not in the center.

Thank you Sir.  Much appreciated.  Bisley hammer won't work in a SAA.  Wish it would.  I've made them before and have one hammer left already cut to make another.  Prefer to buy or trade someone for their's though.  I have some spare USFA factory hammers     So you won't be going with out!

I don't know if this would work but VTI sells a Bisley hammer for Pietta GWII and it says it also fits Colt 1st and 2nd Gen and rmi San Marco.
https://www.vtigunparts.com/store/shopdisplayproducts.asp?id=74&cat=Pietta+1873+Colt+GWII