A 1951 edition of Sir Robert Davis "Deep diving and submarine operations (DDSO)" for $50.00 US dollars? What a great find! I have several copies of DDSO--The last time I purchased DDSO I paid $275.00 US Dollars for the copy! Keep on searching!
The valve on the historic tank was also inported into the US in early 1950s, however the 4 star on and off knob is unique to me..don't recall seeing one in that configuration.
Also the O ring grove is unique- when they appeared on the US market the grove was larger and used a oil impregnated washer, then later a rubber washer for tank to regulator sealing, finally about 1960 the O rings appeared.
Once again a very unique item,
SDM