The standard definition of drachm (in 3 dictionaries I checked) is 1/8th ounce.
I have found "Cheney" spelt "Chenney" but that is not the way Dick Cheney spells his name.
Again mistakes in this puzzle hope people will complain Drachm seems to be the answer and is an apothecary measure 60 grains or Eighth of an ounce. Now as for 50a is a puzzle anyone got a line to the oval office to check it out ! good luck