No problem, just happened to look in this morning.

The capped nipple is ported vacuum source.
Your vacuum advance hose is currently attached to a manifold vacuum source - see attached picture.

(You can always verify the passages and connections for yourself with pressurized air or carb cleaner.)

Urich and Fisher's book is very good introduction with useful pictures and diagrams. Any edition will be fine. At least in the US, a used copy can be bought for much less money.
Urich was Vice President of engineering at Holley in the 70s or 80s.