When did the first 3/4” thread come out on 72’s?

[shurite7]

3/4" with adaptor to 1/2". I have one of those.

 

[iain/hsm]

FYI
I suspect upon close inspection you will discover all sorts of anomalies or possible variations in US Divers SCUBA Tanks up to August 1960 at which time they moved to their new HQ on Delphi Street in Santa Ana and became a almost valid company

At that time they entered into a relationship with Industrial Analyst (IA) to inspect and verify their SCUBA tanks. A small number were rejected for a variety of reasons -mostly inadequate number of threads , but were always still very functionable as a SCUBA tank..
SDM SNIP

I remember some cylinders way back from USD with that 3/4" male to 1/2" female Pipe Thread Reducer (PTR) fitted to give it its proper engineering title. I think the cylinders were supplied before the stocks of 1/2 taper got used up.

The problem from memory was that the thread on the cylinder was a Briggs Thread while the adaptor was NPT we used to make a valve called a Briggs thread to resolve the problem. On the 3/4 inch size both are 14 TPI threads per inch with Briggs its a 29/32 drill size while on NPT its 59/64 to make a minimum of 10 full threads cut. My 1/64th's to the post I guess. Iain

 

[AfterDark]

I have 1960 USD tank that is standard 3/4" it is a pillar valve J

 

[captain]

I have a USD tank from Mar. 58. It has a 1/2" valve but the tank has the larger neck as used with the 3/4" valve.

This is the tank. The neck measures 1.660
Seems plans to go to 3/4” may have begun in 58 or earlier. USD probably waited until stock of 1/2” valves were used up.