Originally Posted by
Beerhunter
I am afraid that is just just rubbish. The 200gn bullet was designed for the Pistol, Revolver No.2 (or Enfield).
You are correct about getting a proper diameter 38 S&W bullet (.361) and not using a .38 Special bullet (.357). However in addition, due to its huge bearing surface the 200gn bullet is indeed very accurate but in revolvers designed to take it. (See above.)
BTW where did you get the date for your Mk.III? They can usually be dated far more accurately,that is to say to the year.