The plunger tube is on the left side of the frame. There's a spring inside that holds a pin at both ends. The one at the rear is the pin that the thumb safety engages and the one in the front is the one that the slide lock engages.
It sits just below the slide and over the left side grip. They are pinned in and can work loose over time. It's not a big fix and don't let it stop you from buying the pistol. As I say, it's an easy fix, but it is
something to check.
We will need many pictures
of this fine specimen and an accompanying range report in due time.