I've been researching this for a couple weeks now and can't find any sources.
The big problem here is the original knob (and the balance pot shaft)
they are the split-spline type.
You can replace the balance pot with a 100K or 250K or 500K (value is NOT critical,
I think the original was 750K) and then use the knob from a PAT4, which is the newer
conventional type with smooth shaft and set screw. The knobs are identical in appearance.
The other option available is to buy 2 mono pots with split-spline shaft, these ARE available,
not great quality but at least 40 years newer than the originals.
You can mount one where the existing BLEND switch is and just disconnect that function.