HP-41 modification: what is this (see photos)?

[Cococollector]

@Allen: EMROOM reveals 753, but not in the expected way (see 1st image). Looks like SW mixed with ED. And I can edit the number to the right, <- deletes and replaces with _, 1-9 replaces _, R/S quits with either new number oder NONEXISTENT depending on unknown condition. ED shows same display except for an R instead of D. With EMDIR it shows the SW face (oh, uh, Apple watch has faces, 41 just a display), which I can only get rid of via ON. R/S does nothing in this state. Comparison:My halfnut shows D001, then X with proper number of registers, here 124.
Found strange behaviour in the CAT section: CAT 1 and 2 deliver properly, CAT 3-6 deliver… the same as CAT 2!
ON/ backspace Sequence leads to MEMORY LOST, but never shows the message itself. I only deduce this from other symptoms, i. e. resetted time, flags, display, size…
Conclusion for now:
additional X Memory confirmed, if unexpected size
I let it sit a while without energy to see whether something recovers.

Edit: MEMORY LOST shown again. Strange CAT behaviour remains.

@KimH:
Where does the C4 belong? On the 41’s board or on the new PCB? Do you have any schematics?
Any idea, what key sequence there could be? What was used in the past, if ever known? If there‘s a key sequence, what use could be the hole of?
I attached a close-up of the PCB area, the hole in the housing is situated over the end of the wire’s red insulation on the lower right.