MS-PL MS style torch connection P Propane L Extended 

MS-PL  = MS style torch connection + P = Propane & L = Extended length.
This is a very old style cutting nozzle designed to fit the MS cutting torch originally made in the 1890's, yes that's how old this style of nozzle is, from the birth of the original BOC company.  The original MS cutter was designed with a very good injector which was required as the only acetylene available was that generated in an acetylene generator.  Coal gas was also at low pressure so had to be sucked out by the injector.  Propane didn't exist when the nozzles were first produced with propane as a fuel only becoming available in the mid 1930s'.

The MS - PL cutting nozzle range is a two piece nozzle with a hexagonal brass inner nozzle paired with a copper outer.  The nozzle inner screws directly into the torch head and has a single central cutting bore as well as the pre-heat slots for the heating gases to pass through.  The nozzle outer seals against the torch head being clamped in place by the nozzle head nut. 

The more modern incarnation of the BOC MS cutting torch was produced by C S Milne and was known as the MilFire or Spitfire cutting torch.