I took the two back hold-down plates off the carriage, cleaned everything up, and re-assembled. There was visible wear on the plates, but not excessively so.
The left (headstock side) plate had two 0.002" brass shims under it, while the right plate had only one 0.002" shim. I removed one shim from each plate, and the carriage dragged somewhat near the tailstock (where the bed is unworn), so I made up two 0.001 shims to replace the removed
0.002" shims.
In any event, the hold down plates were not very loose to start with. The manual shows the late coming with more shims that I have, so prior owners may have been removing shims as needed.
With a 3/8" shank carbide bit, I was able to make a cut ~0.100" deep without chatter, although a heavy hand was needed. It was on the edge of chatter, and a long smoking hot chip came out. I was cutting a shoulder, moving the carriage to the left.
But I do seem to be achieving something like the expected stock removal rate.
Joe Gwinn