This helmet is the rather simple ancestor of the modern helmet, made in former Czechoslovakia in 1983, mostly for the
military purposes. The complete diving set was marked as SP-20 D. The set SP-20 D consists of two wooden boxes (for helmet, neoprene dry diving suit, undergarment, hoses, lead boots, communication set etc.). The diver using SP-20 D had was connected with the surface air supply via the umbilical, there were 2 HP air tanks, 40 litres each on the surface. On the back the diver wore an "come-home" tank of the 7 litres.The whole diving set was only for air use, no nitrox nor mixes. The weight of the whole set SP-20 D was (incl. the 40 lt tanks) 355 kg. The weight of the helmet itself is 11 kg.
There are two possibilities of the air intake to the helmet SP-20 D:

l) use od the 2nd stage of scuba regulator with external adjustment, or
2) use of the "flow thru" system with a continual helmet ventilation.

The working depth of the SP-2O D diving set was within the range of the use of compressed air as a breathing gas. The DM-20 D diving set is rather historical, it is not made nor used anymore in both subsequent states, Czech and Slovak republic.
As an attachment I am sending you the the digital photo of the SP-2O D
czechoslovakian diving helmet. 

Please, let me know whether you receive my message and photos in particular.
With the best wishes,
Frantisek

Many thanks to Frantisek NOVOMESKY M.D.,Ph.D
Professor of Forensic Medicine
Director Institute of Forensic Medicine
Slovak republic