1965 273 4 groove crank pulley questions

So this is the elusive 2 belt AC. Ulf in Scandinavia has 2 early A bodies (from Facebook pages) that have this 2 belt AC system. I have 2 pre 70 small block AC cars that use a single belt (3 pulley) AC system. We have yet to understand what is what in the 2 vs 1 Belt AC. I have personally only seen 1 belt.

So why do you only have a one belt idler pulley? It would be so awesome if we could figure this all out.