Ok, it's a 'pistol' with a brace.

Saw one at a pawn shop with a Romeo sight. Ultra cool.

Now I understand the ATF rules.. mostly.

If one wanted this to still be a pistol then one would have to take the brace competently off, right?

If so is there an attachment that allows a sling in place of the brace?

Thanks!

I don't follow all the sig mpx/mcx permutations and perturbations, but I think this has a 1913 rail on the back. In which case any rail to sling swivel adapter (there are a million of them) will do what you want.