I just got this I guess from the 70's twin set...
I must admit when I saw the picture of it I did not realise it would have a J Valve and when you operate it air comes out of the round holes on the valve body.
I also am not sure what the J Valve would actually do in such a configuration?
Does it turn off one of the tanks or only use one of the tanks at low pressure?
It is now on its way to a IDEST test center which has experience of old kit including J Valves.
http://www.sita.org.uk/idest/
I shall report back more when I get there report...