I have an AAC M4-2000 can that uses a 51t ratchet mount.
But because of the AAC bankruptcy with Remington, the mounts were scrapped and unobtanium.
Happened to Google around today on a whim and found Capitol Armory supposedly has a bunch for 5.56 at less than MSRP.
I ordered three. I think this will be the last of them so get them if you can.
https://www.capitolarmory.com/aac-51...r-556-762.html