US DOT doesn't "regulate" regulators of any kind, least of all scuba yoke valves.
Almost all (obviously there's one exception) 7/8" UNF necked tanks have 300 bar DIN valves on them because at the time of manufacture the entire concept of a convertible valve was new and yokes were not suitable for 3500psi (most were rated to about 3200psi by the manufacturer in the early 90s). The 300 bar DIN fittings were done by convention not regulation. PST needed a "different" valve (than yoke) and 300bar was suitable.
Fast forward about 5 years and PST and others get permission to make exempt tanks at 3442psi with 3/4" NPS necks. Tanks greater than 2600-3000psi have been around for awhile now and yokes have been made stronger in the interim. Valve manufacturers correctly think, "we don't actually need 300bar DIN, 200bar is totally fine. Lets make the valves convertible!" And poof convertible valves are born. Because 7/8" UNF threads were already out of date, essentially nobody retrospectively made 7/8" convertible valves for them, they retooled for 3/4" NPS and that's were we remain today.
If you think 300bar is a USDOT requirement because there is some 3500psi threshold for the deeper threads then cite the CFRs that say so. You won't find any as it has no force of law, its a historical convention.
