Just a follow-up on this topic and some observations spread over a few trucks.
On a couple 1/2-tons I have also seen this vacuum fitting, I believe it was also common to several 1950's Chevrolet/GM car applications.
https://www.flickr.com/photos/advance-design_parts_co/8603092624/in/set-72157633120087261
https://www.flickr.com/photos/advance-design_parts_co/14527050614/in/set-72157631868452754
On the one I have it is stamped with an Old English FP.
https://www.flickr.com/photos/advance-design_parts_co/11593841635/in/set-72157631868452754
The small one, the same shown in Tim's post I have also seen installed in 1/2-tons all on it's own. This piece is a Weatherhead stamped fitting.
https://www.flickr.com/photos/advance-design_parts_co/8255584995/in/set-72157632199330381
The last heavy plug as shown in Tim's I have commonly found on 3/4 and 1-tons, I think the larger manifold hole on these trucks was most likely to make the 213 Series of Hydrovac options easier to install.
The last version on the HD trucks are usually routed to the valve on the firewall for the Hydrovac/2-speed application I have seen a few different styles but don't have many pictures of them
https://www.flickr.com/photos/advance-design_parts_co/8130506223/in/set-72157631870734114

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr