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.