That's a nice addition to an Avo collection Dave.
Never seen one of them before.
I would be inclined to agree with "Wave Solder" that it is
most likely suposed to be used on the 12" Amp Current Range.
If true, that should be easy enough to test with the Set Up you
have already, by measuring the Current of your lamp on the 12 Amp
Range with the Shunt, and hopefully you should see 3.2 Amps.
If not, then this Thread might be getting much longer
Always nice to have a new Avo mystery.
Let us know if you make any progress.
Edit: Posts Crossed
Ian