> Apparently they have investigated using a camera in a box, hung on the
> front of the dairies display, and then using an OCR program to identify
> the number

Roger, I've tried reading an LCD optically (not a camera). It's not easy
or reliable. A lot of unwanted reflection and difficult to pick up individual
pixels

> I suspect the best way would be to 'crack' the communications feeding
> from the reader to the display, though exactly how to do that is my
> dilemma

That sounds like a better plan, but how it's done would depend on the
strength (impedance) of the signal and what sort of radiation you can pick
up. My first approach would be to try a sensitive Hall Effect sensor, to
see if there's any readable electro-magnetism. If that's no good, perhaps
a high-gain inductive voltmeter arrangement. If all else fails, is it possible
to interrupt the cable with a plug and socket and tap into the wire ?

As for decoding the comms, I'd start with known tag numbers and see
what the similarities and differences there are in the message packets.
Might be a format like date, time, location and tag number, perhaps with
punctuation

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