The 'master' is also called the pattern.

"double pairtin' wi' a tap and drag"

I think that might be 'double parting' - which might mean more than 2 pieces to the 'box' that holds the whole

Drag: The bottom half of the pattern

You tap the furnace to drain out the hot metal. A tap as in 'tap and die set' will cut a screw thread into an existing hole in metal, this might mean that the cast piece had to be tapped to make a screw thread in part of it.

I'm a 'Specializing Generalist' - know a little about a lot and a lot about a little.... mucked about with a little black and silver smithing...