Don't know the exact answer but by the 60's or so or the v. early 70's when the last EY's were rebuilt, there was no barrel gauging standard for EY's except that it should be to a comparable standard to that of the sub-standard rifle. That indicates to me that while they could fire ball ammo - as could the sub-standard - if you were really desperate, in reality there would never be an occasion when it would be sanctioned.
I don't remember EVBER range testing any of the load we had. In fact I didn't even know we had a cup to fit to them until I went back to the old workshop a couple of years ago just before it closed for good (it opened as a tank workshop in 1927) and saw the old lathe bed we used to rotate the actions on, a big roll of the wire - AND a cup! Don't even ask.............