Schijnbaar moeilijk aan te komen en duur, maar er is een oplossing :
Batteries The model 7 used a rectangular 1.5 volt cell with a brass terminal and a flying lead for the negative connection. For the highest resistance range two 4.5 volt cycle lamp batteries with brass spring connectors were used.
As a replacement for the latter I have wired in a lead with a push fit connector to suit the more commonly available and cheaper 9 volt PP3 battery.The 1.5 volt battery was replaced with a single cell size D to which I soldered connections.
http://www.richardsradios.co.uk/avo7.html