This is a fantastic and somewhat rare (in the USA) Siebe Gorman eeSGee Mistral one stage British licenced version of the French Spirotechnique Mistral.  The name esSGee is in fact a play on words for "SG" or Siebe Gorman. You will note that this regulator has a Heinke mouthpiece as the Seibe and Heinke companies mereged in 1960.  Siebe Gorman also made a British version of the CG45.      This particular model has the correct pressure gauge in atmospheres and some very well preserved chrome.  My understanding is that this gauge can only handle about 120 atmospheres or 1800 PSI at risk of damaging it.  This regulator is very over engineered from the box clips to the threaded elbows on the horns.  This model has been retrofitted with straight elbows as the original bent ones were a detriment to performance from what divers have told me.