Yas the thread is 20 TPI. I was surprised about that as I thought it was more like 24 TPI but just checked my hub with a thread gauge and 20 is correct.
I took a long while to remove the brake drum on my 101 and soaked it well with WD 40 and also warmed it up with a small diy gas torch. I then tapped it gently with a wooden stick, thin enough to get between the spokes and after a while I could detect a bit of rocking movement. After I knew it was on the move it came off fairly quickly agter that.
I was very careful as I did not know if the hub was cast iron or a maleable forging, so did not want to break it.
When you have the drum off, check the splines as they may well be worn. Mine was bad on both sides, so a new hub was only outcome.
