As a pp said, informally I'd call you Level, more formal I'd call you L 75 (initial and surname), most formal I'd find out your title such as Dr. If you didn't have one I'd stick with initial and surname.
It's rare that I need to initiate formal correspondence having no idea who it's being sent to. If it's professional then my PA will find their details.
The golden rule is to use whatever 'sign off' they use. If their email is sent with Kind Regards, Ms Level then it's what I'd call you.
I think you're unreasonable to to be annoyed at being called Mrs. I don't think you're unreasonable to want to be called Ms. For me, insisting on being called Ms helps build up an impression in the same way we clothes, accent, handshake, hair style and whatever else do. The Ms part is a mark against.
but equality issues do bother me more than most people, and whatever you tell yourself it is an equality issue because of the historic ownership point.
More than most people? How do you score that? At what stage in a relationship do you determine how bothered people are by equality issues. I don't want to be antagonistic (a lot of the time I do, but here I'm trying to avoid it) but slightly snooty statements like that are what I expect from Ms 'people'.
I think historical context is bollocks. In a similar way to the 're-taking' of n----r' or the way woman comes from wifmann (married female) even the most militant MN'ers don't seem to have issues with being 'husband and wife' or being a 'woman'. Look for modern interpretation, intention and context as opposed to being hung-up on historical meanings.