What goes on the rear of an ECZ 9425 B manifold

[OldMercs]

This is on a '54 Merc 292

I picked up an ECZ 9425 B intake. What's the purpose of the rear (firewall) hole? The fitting and cut off hose that were attached are the same size as the vacuum feed for my power brakes.

The second picture is the hookup on the manifold presently on the car; a vacuum line to the distributor.

The third picture is the hookup to the vacuum boost for the power brakes ported of the carb.

My guess is that the brake vacuum should feed from the manifold and the distributor vacuum should come from the carb.

I appreciate your help.