It all seems a bit strange to me that no one really knows what it is.
It's not a real secret. The Agfa
photo (trademark, no producer) APX (with or without the addition "new emulsion") are identical with the Kentmere brothers, both 100 and 400. All Agfaphoto films show a red dot instead of the original red Agfa rhombus. It's so easy.
Agfa and red rhombus = original Agfa Germany
Agfaphoto and red dot = Harman/Kentmere equivalent for bw or Fujifilm color neg and slide films
A bit of confusion (or a lot, to be honest) came with the RPX films, which are also made by harman and at least at the beginning were slightly different, especially the 400 was clearly superiour to the Kentmere. Then the quality seemed to swich to Kentmere 400, then back to the better first batch quality and now again is 100 % identical with the Kentmere/APX 400. I just checked that 3 weeks ago.
So, no matter if you have APX, RPX or Kentmere, you have identical emulsions today.
The APX 100 is a very good film. Of course not as fine grained as a Delta 100, but not "worse" than the FP4+ imho. I get real ISO 200 speed regarding zone I density with Caffenol-C. The APX 400 is mediocre. If you want very nice and sharp but quite big grain, this is you film at 400. But the film seems to be rather push resistant, I don't get more than a solid 400. The first RPX400 was much better in terms of fine grain and pushing behaviour.
I shoot a lot of APX 100 new, available here at a big drugstore chain for 3,50 Eur.
German Agfa (without "photo") closed the doors in 2005. The frozen stock is completely sold out.