1970 A833 issue jumping out of 2nd gear

Shifter in neutral, with drill blank in line up hole. All transmission Shift arms in Neutral. Lengthen or shorten shift rods to perfectly line up with transmission shift arms with linkage rods.

You are not engaging 2nd gear deep enough, so it is riding on the edge of the sincro slider hub and is popping out of gear.

Check for wore out shift forks and replace them with new. Or do the old school fix and build them up with brass and re-machine them back into spec.

Quench shift forks in oil after brazing them to retain hardness.