On Tue, 17 Nov 2020 08:32:56 +0000, The Natural Philosopher wrote:

if that is a straight transformer type with internal regulator,
it
would have a scaled-down voltage reference

But judging by it's size and weight its not got much iron in it...

Doesn't need to have. Not to just provide a voltage proportional to the
mains., Lots of turns of very thin wire on the primary of a tiddly iron
transformer will do nicely...

Only a two wire connection, at 3 V, to the display unit. No "sense"
wire. It looks just like any other generic wall wart supply. No
CurrentCost branding or "use only with...".

--
Cheers
Dave.