It's vacuum, and that could be useful if you want vacuum boosted brakes and a hefty cam, since with the cam you'll lose vacuum at higher rpms.

Wonder if it can be converted to p/s by swapping the pump?

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi