Unknown vacuum line

We are just about to put our 318 into out 1970 Dart. It is a California car non A/C car, original slant six. Looking at the passenger side firewall to the left of the heater blower motor I have a vacuum line that has been cut/broke off before we bought the car that goes to a hard line and goes into the firewall. I have a assembly manual and looked in the heating and A/C area and can't find anything about what that may be.

Any help identifying this would be great.

Thank you!