I have the same peculiarity in my own set. It seems far fewer of the Hoya 81B's sold, and the ones you find were manufactured earlier. I rarely use mine. If I need serious Kelvin correction for especially deep blue shade, particularly at high altitude, I jump from the 81A to 81C, and seldom even carry an 81B. The more pinkish later versions are preferable. KR1.5 and KR3 are Euro designations even a little more reddish than the 81 series. That system is slightly different. For example, if you combine two KR1.5's, it equals a KR3. But if I'm traveling extra light, I might combine a pinkish 2A salmon skylight filter with an 81A to approximate a KR3, at least in hue if not in full density.
Both systems have their own validity, depending on what specific color film you're working with, and what the actual light situation is. So I own both in several diameter sizes, plus a selection of skylight filters.