I searched the forum before I posted. Someone had once mentioned that a couple of Siemens' breakers may be classified for other manufacturers' panels, but didn't give details. And someone gave a link to a list of breakers made by Connecticut Electric, but they're mostly for panels that aren't made anymore, there wasn't any listing for an Eaton breaker.
I asked because just yesterday I needed a 125A BR breaker and had a very hard time finding it. I had checked a couple of weeks ago and found a few in stock locally, so I didn't worry about it, but by the time I actually decided to buy it for sure, the stock was gone.