The 5/16x18 joint on the Helmstetter is the same one that was used on the early ADAM cues. I tried for years to find replacements for mine. After buying a couple of "dead straight" used shafts and cues from various outlets that were anything but straight I had Tony Dahlman at Jacoby Cue make me an Ultra that was not piloted (it's flat), but fits the joint perfectly. The new shaft plays great and stays secure to the butt.
I'll soon be having him make a replacement for for my favorite Huebler (which are 5/16x18 joints, but are flat) and match the ring work.
Tony did a great job for me and the new Ultra shaft improved the hit of my old ADAM cue tremendously, but any reputable cue/shaft maker could set you up with something - basic to high-tech that would work with that joint.
Provided that it threads down securely, the flat 5/16x18 joint is your best bet for a replacement.