I got the A-9A regulator for my A-4 walk around bottle today. there were a few available and I chose this one because it had an extra threaded step down adaptor for which I had a feeling I might need... and I did! (Lucky me!)
I had to put the bottle in the freezer to contract the threaded A-13 so it would unscrew. I heard a slight wisp of 76 year old oxygen escape and pretty sure I saw my 1942 A-4 flight suit twitch and come to life for a split second!
The authentic A-8B oxygen mask attaches perfectly. (as a side note the modern reproduction does NOT fit. It probably only needs the nipple sanded down a bit to slide in though)
now I have a full 8 minutes to get from the living room to the bathroom and back to the couch before I get Anoxia!
Now, unless there was an optional clip on attachment for this, the A-4/A-9A was truly a hand held walk around bottle as the A9A regulator doesn't have a clip like the A-13 does.