Guys - my 1st post, after lurking and looking for a while (I like what I see...
As a veteran mechanic, RickRoush, I'd say you're looking at the axle shaft retaining nut as noted above -- if you've experience with changing the 'Stang pads, you should have no trouble, the TL's pads are prolly easier, if all yer doin' is pad rplmt. (how many miles? pedal gotten spongier progressively? problems stopping/pulling/noises, etc?? given it a visual insp. yet - cond. of rotors, etc...?? car a daily driver/grocery-getter, driven hard, etc??).
A friend's a Honda 'Ricer' and frequents those boards, so installation pics should be easilly obtained. If you're just swappin' pads (not always recommended; see above), only one bolt per wheel must be removed; the caliper then 'swings' away, allowing pad inst.
But be advised, this car has ABS, and if you simply push the caliper piston back into its bore, the dirty fluid will be forced back into the (expensive) master cylinder causing possible damage/an ABS warning light at the least. Check this link for ABS precautions/procedures:
edit: just saw the 'Ricer' - here're pics:
(grrr...the pics wouldn't 'take' -- here's the page & replacement procedure):
If you need further clarification, I'll be ck'ing back tomorrow. Good luck!