They call it the imaginary D version. They want you to think its there but its really not. https://www.ebay.com/itm/1916-d-mercury-dime-Worn-Down-But-There-IS-A-D-ON-THIS-COIN-MY-PICS-ARE-JUNK-BUT/401879530139?hash=item5d91e2fa9b:g:ORwAAOSwRr5ddzYl
Lousy pictures, with the rev shots at a steep angle instead of nearly straight on like they did the obverse. After blowing up the rev pictures I can see something after the E in ONE, but it is not in the right place for a MM and appears to be about half the size of the E in ONE.