These tanks would be marked AG (for Atemluft Geraet) and are breathing air tanks not for underwater use.Doesn't make any difference but if restamped TG (for Tauch Geraet) the interval between reinspections would drop to 2.5 years.
The valvers are "Kleinkonisch" which translates to small taper thread, a system which is no longer being actively supported since there have been several imstances of these valves shearing off when the tank is dropped on the valve.
The current replacement small Tank/Valve combination uses cylindrical M18x1.5 threads and is much stronger.
I'm guessing that the pictured tanks were made more than 40 years ago and finally sold for scrap., after all the firedepartments in Germany converted to 300Bar 35-40 years ago.
Michael