[mark_in_manc]

Thanks for your interest, folks, and thanks mods for sorting out my thread error.

Serial no. is 1205-B-267-M

I think this makes it Feb 1967

The circuit diagram does suggest a resistance on the spring contacts of the battery box, but mine does not have one. Instead, the three R ranges connect (amongst other things) to three component resistors encased in something like blue heat-shrink, with their values printed on.

10,000 works (in my first post I missed a zero somewhere and confused things) - 186 ohms

100,000 won't zero, but the resistor is roasted with no visible legend and o/c. I'll go with making up the value suggested in the diagram above - 1977 ohms - and see what happens.

1000 is dead. Thanks Philpott, I spotted the contacts you suggested which close on this range only and cleaned them. Then I worked out which resistor connects to this range - 15 ohms. Looks OK, but is o/c. So OK, another one to make up.