Not sure anyone can help. This was discussed in the original CS forum. I tried to recover that thread, but failed. IIRC, there is very little detail on the EP round and even less on the EPS.
Cappellano states the EP was a chemical energy warhead with a rear mounted fuse. EPS-42 used a special spherical primer mounted on a warhead of German design (p. 260). P. 296 states a unit of fire was 250 rounds; of which 2/3 were ordinary and 1/3 was EP.
It weould just seam reasionable to assume that if the gun was deployed "up front" in a direct fire role either defensively or offensively that they would have on hand more AP or EP's rounds or as many of the later as they could lay there hands on, but early in the war given the fairly week nature of British armor, except the Matty, the standard AP round was effective, its that the above allotment of ammo is a bit vague.
While it isn't specifically stated in the source, I have always assumed that the unit of fire info is for 1943, so later in the war. EP was generally not available until 1942, so any UNFOC table with EP can't be from 1940-41. I would assume that the 1/3 EP would be met by either EP rounds, or if not available, proietti perforanti (AP). So 2/3 HE, 1/3 some type of c.c. round (PP or EP).
In 1940, it is like the same UNFOC was in place. Note that the UNFOC is not necessarily what was carried with the gun. UNFOC is a planning tool for anticipating consumption in a theater. If the theater command decided that the UNFOC on hand in A.S. is 4, the Intendenza would need to maintain 667 HE and 333 PP/EP rounds per gun in theater (a total of 1000 ads, or 4 x 250). If the planners decided that they will expend 10 UNFOC over the course of a future operation, then the Intendenza would need to requisition/stockpile a total of 2.500 round per each 65/17 gun in theater.
A basic load is what is carried with the gun/unit trains. A division commander might decide that the mix of HE and PP is different from the UNFOC as his plan might anticipant the guns doing more/less firing of certain types of rounds. A 65/17 operating in the mountains likely doesn't need PP.
EP was available in spring 1942 and was issued to units in Russia and A.S. (Cappellano p. 259).
While its my understanding that was more or less universally true for several different guns, what's confusing is that it was available according to what I have read both on this forum and on AH's forum that the, for example, 47/32 had it in late 40 and some others around the first part 41 (?) Unless I am mistaken ?