Dear Gabriele,

I don't really have an answer to your question, just to note that at the
beginning of the Sgra-sbyor Bam-po Gnyis-pa (Madhyavyutpatti) in the entry for
"Buddha" you can read these words: gti mug gi gnyid sangs pas na mi gnyid sangs
pa bzhin te / sangs pa la snyegs ... blo bye zhing rgyas pas na padma kha bye
zhing rgyas pa dang 'dra bar bshad de sangs rgyas zhes bya'o.*

(I only quoted the relevant parts of the entry, but you can see that its
explanation of the sangs and the rgyas is the same... Try word-checking through
the Bodhicaryavatara commentary by Vibhuticandra in the Asian Classics Input
Project website, too.)

*I found this in my copy of Nils Simonsson's "Indo-tibetische Studien" (Uppsala
1957).

Yours,
Dan