Good grief! The word pedantic just about covers it [did not know that word was used in Texas..

.]
Seriously everybody has one....an opinion....even the "experts"....at least in 1955 the USD catalog did originally simply refer to a non-reserve valve post in a single steel tank and a reserve valve in a single steel tank [72s].... commonly called then a "K" valve = non-reserve.....while the "J" valve = reserve valve....
.yes, a reserve....it usually held back bottle pressure of 300-500 psi in reserve until the "J" valve was pulled down or pushed up to access that last 300-500 psi in the tank [you could configure the J-rod to open/access the reserved air by pulling, most common, or pushing the rod up]....why? Cuz kelp/etc. could accidentally pull the J-rod down and you would not have the reserve gas you were counting on....counting on? Yup, we dived with two-hose regs [DA] and when the breathing got hard [without pulling J valve you ran out of gas] we knew we were near the last available air unless we pulled the J-valve...normally for our ascent...no SPGs then...pre-gadget old school dive planning.....obviously all this was using yoke attachment for regs to valves and no "DIN"s in use.
How do I know the above? Because I first started diving in 1955 and we routinely dived deep, got hard to breath [DA reg], pull J rod and ascend....extra 300-500 psi plus less hydrostatic pressure on ascent and breathing got easier...all good and nobody died..

Contrary to the next WAG I can foresee...in all these years, I still use J valves, I have never know of or had a J valve fail...
Why all the bloviating about such a topic escapes me. Interested in diving history? Good. But to simply lambast the OP or anyone to prove them inaccurate seems to becoming the norm and only diminishes the value of the information exchange.
Ps... Interestlly some old two hose regs had built in "J" valve type reserves which also required pulling to activate...