gser phye long mo gang?

Ms. Mengyan Li, my doctoral student, and I have encountered an expression gser phye long mo gang in the Phur pa chos ’byung she is studying. From the context it is absolutely clear that long mo must be a unit of measurement, that is, something like bre. This is, of course, assuming that there is no scribal or transmissional error. To be sure, there is no Varia lectio. The reading long mo in the sense of anklebone seems to make no sense. I think long mo gang cannot be equal to bre gang. gNyags was undergoing hard times then and he was not rich. We tried to look for this word elsewhere but it seems to be what is called a Hapax legomenon. I suspect that long mo gang should mean something like spar gang or khyor gang but there is no way that we can verify or falsify.