Yea you are right, I would say 1990s based on the wattage rating being on a metal plate embedded into that plastic. Not newer than 2000 unless it is some very specialty company.

Yea all of them should be able to pull off because the cover cannot go on with the knob on there. However there could be a screw in the side holding it on, can’t see all the way around it. If your mom is old she might just have shit grip strength and can’t grab it to pull it off. I know plenty of older women that have what I’d consider no strength there at all, that is all forearm.

Usually you pull the knob and it is removed. But being that old it might be stuck or break. You need to remove the knob before the switch plate.

They still make that style so it could be brand new or it could be 60 years old.

That style should be clutch fit, i.e. the knob just pulls off. It may be gunked on there with 40 years of accumulated crud, though. So this is likely to require a hard yank or possibly some prying. Good news: The cover plate is already broken, so prying on it can’t possibly meaningfully break it further.

It’s also not outside the realm of possibility that her handyman broke it, and his solution for “fixing” it was to just glue the knob onto its stem before anybody noticed.

If it breaks, no big deal. Breaking it is also a valid way to get it off, and an entire replacement dimmer is like $9 at the hardware store. You can also get replacement knobs for a couple of bucks, and they’re generally broadly interchangeable (although these days, without that groovy aluminum accent disk in the middle).

Break it. Replace the switch.

Looks like a tiny screw hole on the top.