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Accuracy International AT-X

There seems to be no real standard on the fasteners.

Nonsense. There is an explicit set of standards for MLOK. The wiki page has a pretty good overview but you can get the entire standard from Magpul. https://en.wikipedia.org/wiki/M-LOK

Whether certain manufacturers adhere to those standards is a different story. Ironically, to my knowledge, AI has never published the standard for Keyslot. But MLOK absolutely has a standard for dimensions, fasteners, torque, etc. It has a full TDP.