That's a very old Hapco mounting arm, I believe from the late 50s to around 1960 or so. Sometime in the early 60s they switched to a welded on fixed-position lag bolt flange. The lag bolt flange above is bolt-on and can be adjusted up and down the arm to adjust the leveling of the arm if the pole isn't straight. Since virtually all fixtures had their own means of leveling via the slipfitter, I guess this feature was seen as redundant and done away with for cost-saving measures.
I've seen some of these arms end up leaning sideways or sagging downward because the lag bolt flange bolts loosened up. Seems to be fairly common with the arms here, especially the 8ft versions. The ones here originally held remote ballast MV lights so they probably weren't designed to hold the weight of the integrally ballasted 400W MV cobraheads that the electric company here installed on them in the 60s and 70s.