Yeah, I, and a bunch of other people have assumed at one point or another that the order was reversed, since that makes the most sense- the mandate for 5mph bumpers took effect and there was a mid-year switch. But because of some kind of production issue in the front, it was actually backward with the '74 model year starting with shock bumpers and then going back to standard, non-shock mounted bumpers in the front only.
Pretty much all of the cars I've heard about fit into 2 categories, August - December '73 build date '74's that had shock mounted bumpers and the single mounting hole, and then December '73 through to July '74 build date '74's that had standard brackets and 3 holes in the frame. There's been one exception I'm aware of so far, and that was @DionR 's car that had shock mounts, 3 hole locations, but the standard hole locations were unthreaded. His car was a late November '73 build. So likely the factory knew they had to change over and the first change was the frame rail mounts, until they ran out of shock mount bumpers. So if I were a betting man, I'd say your parts car was probably built late November to early December to have shock mount brackets and all 3 frame rail locations.
It's been covered a bunch of times, there are a few threads on it with '74 owners posting the build dates of their cars and the type of bumpers they had that established the general dates I've listed so far. Obviously no exact dates for the changeover just based on '74 owners listing their dates, and like DionR's car I'm sure there are some different combinations close to the changeover dates.
Here's a few of the threads, there's others linked in these too
How many CURRENT owners of real '74 360 Duster 4 speed cars on here?
What the rubber bumper is going on in 74
'73 bumper brackets on a '76 ?
