On US-65/Iowa 330, there are these markers (these were taken on the SB side, but they're posted the same on the NB side too):
(https://i.imgur.com/324IEko.jpg)
(https://i.imgur.com/iaVAI9F.jpg)
The two markers pictured are definitely less than a mile apart. In addition, further south there is another Mile 96 marker without anything beneath it. What's the purpose of these markers, and what do they mean?
Ahead and back. The highway was realigned and lengthened, so there is now more than a mile between 96 and 97. I don't know all the details of how it works.
Good to know. I figured it had something to do with realignment lengthening but I could not, for the life of me, figure out what AH and BK stood for.
Now I guess my question is: what do "ahead" and "back" mean specifically in this context? Why not just have a sign abbreviation stating "long mile?"
The ahead and back usually indicates the specific point of mileposting change from the realignment. When following the direction of increasing mileposts, this denotes the difference in mileposts from what lies behind you ("back") from what lies ahead. This specific point is typically referred to as a "milepost equation".
For example: Say a curvy highway was reconstructed between mileposts 20-23 such that the alignment was straightened, resulting in a half mile reduction in highway length. Original milepost 20 would still be the same. At original milepost 23, you'd likely find two signs: one would read "MILE 23.0 AH" and the other would be "MILE 22.5 BK". This would be a field indication that the length of highway was decreased by 0.5 miles.
For the depicted condition, the ahead and back notation isn't always posted in the field like this, and they're usually posted at the same point... Since you were heading southbound (opposite of increasing mileposts), you first encountered MILE 97 AH, so that's a return to original mileposting. It's a bit harder to tell on the rest of it without the construction context (or knowing what the engineer/DOT policy is on realignment mileposting). My guess is that MILE 96A BK is the original Mile 97, and the distance change occurred between there and current 97.
It looks to me (just very casually, also in ignorance of Iowa DOT's actual policies for dealing with mileposting along a relocation) that an extra partial Mile 96A was created to accommodate a slight lengthening, possibly due to construction of a gumboil-type bypass where the old route went straight through town.
I've seen these throughout Wyoming, often for minor realignments and sometimes for major realignments. Here is an example of how Wyoming signs these, as found along SR 130 near Centennial. In this case, the realignment resulted in a reduction of 0.33 mile (27.75-27.42=0.33).
(https://www.aaroads.com/west/wyoming130/wy-130_wb_centennial_03.jpg)