vacuum port on base of 4100 - ported or manifold?

Perhaps this is a dumb question. Is the vacuum connection on the base of a 4100, on the passenger side, a ported vacuum source, or a manifold vacuum source?

I have that port connected to my distributor vacuum advance. At idle, the timing is advancing about 10 to 15 degrees, which suggests either 1). it is manifold vacuum or 2). there is something wrong b/c I understand it should be zero at idle.

Tim