I've got a documented Mach 1 made for export (the T5 package) that was built in the same plant on the same day as yours. The consecutive serial # is also in the 115s. Pretty sure it came with the original grille when I bought it - no place for an emblem. I bet it's aftermarket and/or modified.
On a side note , these were definitely early build 70s. They've got a few features carried over from the 69, such as the glued on windows (vs bolted), and a more complicated headlight wiring harness.