There were many versions of hydraulic conversions to the standard disc brake.
Most screwed in, but it is not clear from the image you posted whether that is the sort you have, although you say it looks to be that type.
If you can see a thread on the slave cylinder, then it will be.
The ‘World & His Wife’ may have made yours, but hopefully, it will prove to be a good version, worth saving.
Until you establish whether it is a screw in type & if so remove it to strip it, it is not possible to tell what make it may be. With luck, it could be an MSC which were the best, in my experience.
Copious amounts of penetrating oil (NOT WD40 or GT85) will help after soaking for days. Gentle heating of the housing (NOT THE SLAVE) will also encourage it to free up.
On one of mine, I machined two flats so that I could hold it with tools, but I made sure one of the flats served the dual purpose of giving the additional clearance needed to the damper when braking hard & the front wheel is hopping….
When you come to re-assemble, don’t let anybody tell you to leave the slave not fully tightened & free to revolve in the threaded housing as was the practice years ago. Many of us did that but it fractures the housing. It’s best to use a low strength Loctite & screw in until it bottoms out.
It sounds like you may have early wide rims. Before being commercially available, racers used to convert V*spa rims. The very thought of it makes me shudder….
