It is time to send a box to Paul --
I note that I have accumulated a number of heads. The Berea ones use 3/16 shaft, seem to have good steel, and were sharpened with good gear. I have a number of heads that I got cheap -- steel does not appear to hold an edge as well, but hard to tell for sure as the grinder had the grit of a chainsaw cut.
I have a couple of what appear to be older PSI or similar that fell between the Berea and no-name versions in terms of functional life. Attempts to hone edges have been done on all of them -- but after a bit, without the controls of a tool grinder, one tooth get to doing all the work.