NF - Enforcement question

[That Don Guy]

I read a question somewhere about an ambiguity with the new horse collar tackle rule, and I noticed there was a situation where I was not sure where it should be enforced. (Note that I am not an official, but I figure this is the place to ask rule questions.)

Here's the play (which, BTW, has nothing to do with horse collars):
A's ball on the 50. A88 carries the ball to B's 24, where he fumbles. While the ball is loose, B66 grabs A1's facemask (on B's 20). B40 picks up the ball and returns it for a touchdown.
Assuming the penalty is accepted, whose ball is it, and where is it enforced?

Somebody claimed that it was a "loose ball play" because of the fumble.
I am under the impression that it's not a loose ball play as the fumble was beyond the line of scrimmage.
All-but-one doesn't seem to cover this specific situation, and it's not in the case book either. I would say that, since A was last in possession, it's A's ball, and it is enforced from the spot of the fumble, so A's ball on B's 12.
Is this correct?