Just to start a little further discussion: As an old folk musician, I once purchased a button accordion which apparently came from Germany and which I though was in F Major. After inspecting it some time after I had it, I discovered the Key was stamped on the bottom end as E#. Accompiment seemed to be fine using F Major on other instruments. Any thoughts?

More likely what was stamped there was "Es", the usual German way of writing E flat. An E flat button accordion would sorta work for accompanying tunes in F, up to a point.