The scales are good for 12 & 24 vdc measurements and can cater for up to 30 amps, but only low resistance values. Meters for electronic use tend to have decimal scale values to 100. A switch in the centre can be set to double the scale value for all ranges except ohms and it has the usual overload cut-out. The instructions for use are riveted to the back. Mine has the AM (Air Ministry) insignia at the top of the dial, but War Department arrows have also been sighted. The meter requires a 1.5V battery for use on resistance measurement. Mine has an Eveready rectangular model with two screw top connectors. For use today, adapting to fit a "D" cell would be the best solution.
As with most AVO meters, individual units can be dated from the last 3 digits of the serial number under the right hand end of the scale arc which define the month and year of manufacture. For example No 99999-843 would mean that the unit was made in August 1943. Most of them appear to have the scale figures marked by hand to suit the individual meter.
Often the engraved markings on the knobs lose their paint over the years. I find a good method of restoring these is to re-paint with matt white modeller's paint, such as Humbrol. You don't need to be too careful as you can wipe off the surplus with a rag with a bit of mineral turps when almost fully dry. This is harder to do near a crinkle finish when surplus paint removal can be rather difficult.
One tip when buying AVO meters is to check whether the meter movement is working. This can be done without any equipment by rapidly rotating the meter a horizontally to and fro to make the pointer move on an amps range. If the movement is OK it will quickly return to zero with very little oscillation due to damping resulting from a working coil. If it doesn't return to zero or oscillates then it is probably faulty.
As far as collectability is concerned, these units are relatively rare. This may be because, according to rumour, most were destroyed after hostilities to avoid killing the market for new meters. In addition, their ranges make them less suitable for hobbyist use than the Model 7 or 8s, so if you see one, it is unlikely to be expensive.