WW2 British pyjama jacket

I had these also in the U.S. version, but now it is a British one.
It is a WW2 British short sleeve pyjama jacket.
Has no collar and is from the slip-on type.
There are 3 buttons to close it to get it warmer.
On the back there is a black ink stamp that reads “^ R.M.W.2”
Don’t ask me what it means, because I don’t know !
Jackets have some dirt from storage.
Markings:  see text and pictures
Condition: mint / bit dirty from storage