Well... after a loooong time, I've finally got around to looking at this again. Thanks for the suggestions in the meantime.
I ended pulling apart the main unit (goodbye warranty
) to look at its loopback circuit.
It appears to be identical with the exception of R12, R11, R4 and R15 below (unit #1). They are double the value in the loopback.
This triggered a memory of a pattern I'd noticed earlier in unit #1:
The ratio of the the resistor leading to the diodes to the other resistor(s) that the diode is across is 1:5
e.g. R12 * 5 = R2,
R4 * 5 = (R3 + R8 + R5),
etc.
The ratio of the loopback unit is then obviously 1:10.
So I've ordered a bunch of resistors to suit ratios 1 through to 10; more experimentation to follow once these are received.
In the meantime, production of identifiers using parts I already have has begun......