Certainly that series of numbers does not show up in PH catalogues that I have seen. I have a number of the 4440V bases here - appear identical size and shape to the BA28, except the more central screw hole is relieved to accept a recoil divot from a ring. If they are interchangeable with A28, that would be the rear base of a Mauser 98 rifle, with the charger lip removed. So the partner front base, for a Mauser 98, would likely be 4440P, which appears to be identical in all ways to a BA 22. I also have rings with that 3901A number - the ones I have appear to be identical to the RALS.3 or RAHS.3 rings - I forget which height they are. I do not know why the series numbers were changed, nor have I ever found a catalogue or listing calling for those numbers for any particular rifle. I do not know if that makes them "rare", so much as "odd"???
EDIT - looking in my "stash", I also discovered a number of 4440C bases,which seem to be a bit taller than the 4440P - so perhaps, one or other goes on 98 Mauser and the other on a small ring mauser. At the moment, I do not know. I would have to use micrometer on them to find what they are the "same as", then use that catalogue listing for that "same as". FYI, on the rifles of that time - like PH sporter mausers and BSA, I find that the grinding and finish was not exactly always the same - so even using the correct catalogue listing of bases, does not guarantee that they are aligned flat on top - I have had to shim many. Most definitely was not "CNC tolerance" manufacturing and finishing in those days!!!
FURTHER EDIT - if you are looking to sell those, you probably would need to identify the rear base (4440V?), the front base (???) and the height (??) and diameter (??) of the rings, and since the information is so difficult to find, to also state what rifle(s) that assembly was made for. The measurement need to be accurate - a 1" ring is 25.4mm; that is a ".3" in the other PH system, and is different than a ".4" which is 26mm. They do not properly interchange, even though only 0.6mm difference in diameter.