Hello Gelbster,
in general this screw won't come loose, but as Gilles mentioned the gearbox had an overhaul and got replaced pinion and mainshaft bearings and 3rd and 2nd gear synchros (along with a OS Giken LSD).
Number 34 on the lower photo is fixed gear wheel of 3rd gear. Number 20 is shift rod for 1st/2nd gear.
To replace the synchros of 2nd and 3rd gear they needed to disassemble the complete gearbox. So maybe this was the time when the fault was built in. But that is only a guess – everything can fail sometimes.
Sorry, can't offer a link.
Regards, Markus