Yeh could be a possibility! It would be easy enough to then take bits off the working one.

Only the odo not working ?

If the speedo and rev counter are working correctly, that means the signals coming into the cluster must be good.

So it's either that board or a bad signal from that board to the main cluster board.

Occasionally on my 90 the speedo drops out and this also stops the odo from turning

Yep, just the odo and trip not working.

There is one on eBay for a b4 2.0 16v, would that matter??

My guess is it should be ok, but I am not 100%, theres others on here that could tell you for certainty. I was swapping one from a 2.6 auto to another 2.6 auto.

Have you got a 160mph speedometer on yours or a 120mph version ?

It's the 120mph one, the part number is 8A0 919 035 F, I see the one on eBay is 160

That's a problem as the 120mph versions are usually UN4 but the 160 will be VDO

What do these refer to Mike?

Also from odometer gears page there's a bit there that talks about the K value... I'll quote it..
"The yellow square indicates the data tag for the speedometer unit. The CQ tag is listed with a K=6765. This number is the number of pulses per mile. Using this number a frequency can be determined for any speed. This is helpful if you need to pulse a new unit so that the speedometer reads the correct odometer. This K value also allows you to get a used cluster that will work properly in your car. The CQ K value is different from all other B3 models and is also different from the
B4's."

I wonder what the 1.9tdi clocks would be, and would they be compatible???

I'd not come across UN4, but VDO is an instrument manufacturer, so I guess UN4 is code for another supplier. Most of mine are VDO gauges.

As for the pulses, beside dietary advice;
Pulses per mile, gives a pulse 'frequency' when combined with a time reference. That is then converted into voltage, which drives the pointer.
The same pulses being used to drive the odometer

(As an aside, there is a configuration card which is installed in the instrument cluster which sets it to run metric or those other units that confuse the Europeans so much.)