amn't is also found in dictionaries, obviously mean to pronounced [ahnt] (the same as an't/aunt/aren't).
My mother often used "amn't" in a joking manner (pronounced with all the letters, since we are in Canada) and I occasionally use it in informal conversation.

I have also heard (in UK) "amn't" with all the letters and have also used it myself without really knowing why I did so. I have obviously seen it written, but have never questioned why they or myself used it.

In some UK dialects you will hear the "r" pronounced in "aren't".

Rod
ta-daaaaa!