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. It 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.]