Probably the most popular typewriter people harvest from are Triumph Adlers for their caps (MX compatible). Other than that, I'm not sure how many electric typewriters use MX switches or which ones they are, many had older cherry switches like M6/M7. One that comes to mind is the Royal Satellite III, which can have MX blacks. And there are some that have Topre compatible caps. There are also some that have tee mount alps like the Sharp ZX-515