Early/earliest HP-41CV halfnut

[AndiGer]

These days I received an HP-41CV halfnut with serial 2405S...
The earliest halfnut serial here https://www.hpmuseum.org/collect.htm#earliest
is claimed to 2511 ...
So I asked the seller. He told me he bought it new and it was neither serviced nor did he opened it for repair (what I think is true due to the well attached rubber feet).
He used it until 2019 as a daily driver. Unfortunately he then left the batteries in and they leaked. I opened and cleaned the corrosion but it doesn't work correctly.
After a MEMORY LOST - which comes up after about half an hour of batteries out - and left after about 15 seconds on it suddenly display 1/X then X<>Y followed by NULL - as if you were depressing that key constantly. After this happens the only keys that show a reaction are the keys in the same keyboard row i.e. X<>Y, RDN, SIN, COS and TAN.
When I press back arrow and ON it immediately shows X<>Y followed by NULL. Sometimes I can switch to one of the keys of that row and after back arrow + ON it shows that key followed by NULL.

a) Anybody knows what to check / repair?
b) This is really a very early serial?
Thanx
Andi