hmmmm, well I have a 72 Mach 1 as well and this is what the back of mine looks like.
I don't remember ever seeing holes for one during the restoration either, so it may have had a metal "mustang" name logo on the back like you said if it was not an original mach, or if it was a replacement trunk.
I know w/ ours one of the fender panels had been replaced and did have the 3 holes for the mustang name logo of a regular 71-3 mustang (though w/ ours we filled them in)
Really,the best way to figure out what you've actually got is to check the VIN on the dash, and the data tag on the drivers side door. The VIN will tell you the general info
(year,plant built, type,engine,etc)
And the data tag will give you info more specific to the car itself
(exterior/trim colors,trans,axle,original dealer location)
Basically for a 72 mach, the VIN should start w/ a 2 for 1972, then either an F,R, or T for the plant it was built at, (for example mine has an F for the dearborne plant) and then have an "05" to tell you it's a legit Mach 1. Otherwise you will probably see an 02 if it's just a fastback/sportroof.
Also the engine code will be a LETTER....for the 351's it will either be a Q,R, or H. For an example,mine is an H because it was originally the 2 Barrel Carb 351C (though we upgraded it to a 4 barrel) if you see an F or L it was not originally a 351