There it is, big as life, on the cover; the sigil of the Order of Hermes: 3 entwined golden snakes with an ankh, a roman symbol of power (don't remember its name) & an apple - each on the a different snake's forehead. The snakes are all facing an upside down triangle, all equals sides.
Actually, the guy you probably need input from is Richard Thomas, the cover artist and logo designer for the original Order of Hermes, which is where the symbol first appeared.
I was thinking that Mssrs Rein-Hagen & Tweet would have given the "final go" on anything Ars Magica (& thusly would know exactly what everything meant), or even more specifically, Mr Nephew... Reasonable?
Givewn how things were done back then the "cool drawing! let's use it!" approach is much more likely than a deeply thought analysis of the iconography.
For the record, I have ended up prefering the former approach.