REG. HOSE MR22 1/2INC D.5.5

This regulator hose has a  1/2-inch male fitting on one end for the oversized low-pressure  port found on a few first stages, along with a standard 9/16-inch female fitting on the other end for a second stage.

In the 90's some regulator manufacturers (notably Apeks, Aqualung, and Mares) started offering some versions of their first stages with an oversized 1/2" low-pressure port. The oversized port has an optimized gas flow path and should be used to supply the primary second stage. (Mares calls it a DFC port.)