Masbovalsen is a ubiquitous waltz in the repertoire of northern Uppland. In any gathering of fiddlers and nyckelharpa players there, it is one of the tunes most likely to get played. It being in the key of C makes it a little awkward on the fiddle, but it is very easy to play on a nyckelharpa in the traditional C tuning.
Here is an “allspel” from the Rapp-Kalle festival in Uppland in 2010. It is a very good example of musicians coming together for fun, and performing on stage without rehearsal.