Woodstock with serial number in battery compartment

[Harald]

I just received this HP21. It had pretty bad corrosion from a leaking battery. But it was fixable. All the ICs survived and the green/blue stuff stopped just before getting to the display.
Somebody tried to pry out the battery and did not succeed. Luckily without doing too much damage. It was easy to open after removing the two screws under the rubber feet.

But now we get to the reason for this thread:
Which Woodstocks had the serial number in the battery compartment? It is my first one that has this.