Bike Forums - Weird early 1990s Tange threaded headset with no spanner flats

Hi - here's a puzzle from a fairly inexperienced home bike mechanic. I have an early 1990s Raleigh road bike that I've been using as my everyday town bike for about 15 years. In that time I've replaced pretty much every component on the bike, save for the headset. I've never touched it, because I don't understand it. This has reached the point where I am beginning to worry that it's a ticking time-bomb. At the very least it needs cleaning out and re-greasing. More likely, it probably needs replacing.

It appears to be a standard 1" threaded headset, *except* there are no spanner flats. In their place is what looks like a thumb-turn. (I’m not allowed to attach pics so if someone can DM me on here who *is* allowed, I’ll email them the pic to post as a reply). It's a type I simply have never seen on any other bike I've ever owned or serviced. I've tried Googling to no avail. To this end, I have a number of questions:

1. What type of headset is this? Does it have a name?
2. How do I remove it? Are there guides online or in books? Do I need some obscure tool, or does the 'thumb turn' look indicate that it just unscrews by hand?
3. If I can get it off, can I replace it with a standard Tange ISO 1" threaded headset? Or do I need to source a like-for-like replacement? (If yes to the latter - where?)

Any advice very gratefully received. Thanks!