It's hard to describe who's tune this is and I can only refer to it as the 'commonly used' or 'popular' tune for birkat hamazon. I seems like it is compiled of many unrelated tunes. At times there are melodies for a specific phrase and at other times there are melodies for paragraphs.
Was there an 'original tune' that has now developed and where was it from?
[I'm interested in this, because I dislike the melodies (!) and of course wish to implement universal changes at some very near future.]