Forster has been out of the O-rings that go on the cutter shaft and keeps the shaft from sliding off the trimmer for the better part of a year. Its a thin O-ring and I've not seen an aftermarket source. I've got a couple boxes of assorted O- rings and they are all too thick.

Anyone know the specs for this or where I could find a few?

Why don't you pull the cutter out of the unit, take it down to the local hardware and get a “real” retainer ring, snap or E ring. A rubber O ring as a retainer is pretty cheesy. Sounds like something Lee would do.

Did exactly that about 10 years ago and still chugging along.
George