The most detailed, systematic, impressive, and rewarding presentation of skung yig—which I am by the way wont to translate, not simply as “abbreviation” (as is often done) but as “orthographic abbreviation”—is given by Nor-brang-o-rgyan in his Bod kyi skung yig gi rnam gzhag chung ngu. In gSung rtsom phyogs btus. Beijing: Krung-go’i-bod-rig-pa-dpe-skrun-khang, 2006, pp. 1–41. One can learn a great deal from this scholar!