When looking at the MCX's handguard it's pretty easy to think that the front non Keymod hole is a QD mount. Not so much. Put a QD swivel in and you will find it does fit however the locking bearings cant lock into place because the handguard is just a little bit to thick. I contacted Sigs CS thinking they made a machining error only to find out that this hole was not designed as a QD mount at all. It's designed as a recoil lug for their pic rails. Interesting because that's the whole idea behind the Keymod design itself. A pic is worth a 1000 words.
When asked why it wasn't a QD mount, it came down to strength of the handguard. Don't like it but it makes sense.
This is Sigs solution for a QD mount.
Vey very cool. A single slot mounting QD mount. Their testing shows that the rail will fail before this mount.
-M