If it had all 3 mounting holes it probably came from the factory with a non-shock mount bumper. I kind of assumed that the factory had planned on returning to the shock mounted bumpers at some point, which is why the mid to late '74 cars got all 3 holes, so whenever the production on the shock mount bumpers picked back up they could just add them back in without a frame rail production line change.
Mine was a June build, and it had a non-shock front bumper but all 3 mounting locations as I showed above.
I converted the rear bumper over to standard bumper brackets too, first with some home made brackets and later by welding up the rear pan and using the earlier brackets.
Glad to help! But yeah, if you want to swap to a non-shock mounted bumper it would be best to get the bumper and brackets as a set. Because the '73 bumper was also different from both of the '74's. So it would be really easy to end up with a mismatch and have to fab some mounting locations on the brackets