That's a standard blue steel valve. Here's yor next quandary. Do your yokes fit on them? See, they are really designed for DIN, and with the converters in them, many yoke regulators have too small of an "a-clamp" to go on the valve, which is also why your caps don't fit them.
Your solution, as rhwestfall said is to use 1" blue painters tape on the face of the valve to indicate it's state of fill. If the tape is still on, it's full. If the tape has been removed, it's something besides full, or may be full too.
It's what all of us coolio tech divers do. Folks will mistake you for a coolio tech diver. Bonus.