Yah it must be threaded. Don't see any other way it's being held on. A couple IH CC web sites show a pic of the adapter but not how it goes on. Flywheel rotation is clockwise looking at it from back side of the engine so the adapter should be lefty loosey. I don't wanna bugger it up in case I put the engine back in the 1200 so I'm a bit reluctant to use a pipe wrench. I could use a couple feet of electrical tape to mummify the outer edges of the pulley groove so it doesn't get chewed up I guess. Could try a thick belt wrapped around it too.
I tried my antique impact driver on the flywheel screen screws. Doesn't get used much. Not as mint as I remembered it but still in pretty good condition. I gave it a go because the bolts holding the screen on were Philips and wouldn't budge, and the impact bits were the right size for a change. None of the screws moved a hair though. All I accomplished was to pinch my left hand index finger in the spring several times (I'm a slow learner).
So then I used my patent pending chisel trick. Punched a small V notch in the screw head on one edge then used the notch to tap the screwhead sideways to the left and loosen it enough to use a regular Philips screwdriver. Worked for all of them. I did that on the 140 screen too but they were smaller diameter screws and the heads peeled off instead of the screws turning. Had to drill them out. Worked out ok but removing broken bolts/screws is always a PITA.
Edit: added fancy chisel graphic.