Is this a PSK Enforcement?

[jrfath]

Play from last night...

4th and 13...K is punting.
R brings out their return team...I count...think I have 12 R's, but am not positive. I should have flagged it, counted to make sure, and then it would have been Illegal Substitution (5 yards). But I am unable to confirm 12 players before the punt.
K punts the ball...during the punt, I am able to recount, and R does indeed have 12 (DRATS!). So I drop my flag.
R ends the play with the ball.
After the play, I tell the referee we have 12 R players...therefore illegal participation.

IS THIS A PSK ENFORCEMENT (go to the end of the kick, penalize the 15 yards on R, and it is their ball 1st and 10)?

Or it is enforced from the previous spot like a loose ball play. Penalize R 15 yards, which in turn gives K a first down?