I believe you are right, the International Electrotechnical Vocabulary IEC60050- part something should cover it, but good luck finding the section that covers it among the hundreds of parts. I would be surprised if it defined in detail the distinction you are making. It seems traditional to put the higher voltage first - eg 440/250VAC (though there are plenty of examples the other way round), and I don't think your last example with lower case is preferred to upper case.
if you are asking in relation to labelling a product, or preparing the documentation or user manual, it would be worth talking to the Notified Body doing approval for you, (if that's happening)