Hi Pete....Welcome to the World of Theoretical Physics. The ATD retaining bolt simply unscrews from the unit (dream on)...it has a left hand thread, the so called self extracting mechanism, but they are usually reluctant to come out...like I say, theoretical. Disabling the ATD is usually done by tack welding it up, but this is a bit of a one way trip, so only do it if the ATD is well worn and knackered, they still have some good residual value.
Better to treat yourself to a new or used quality (?) fixed drive gear and a new retaining bolt.
The Frankenstein Nuts I have seen were cheapskate homespun drive tags welded to the standard retaining bolt, a bit ugly, hence the name. If a genuine part exists, it will be a bit of a search.
No doubt Julian will be along soon, to shoot me down in flames. Probably got the left hand and right hand threads confused.
Swarfy.