volquartsen LLV "upper"

So I picked up a volquartsen llv upper from a member here, the old style. I love it on my 22/45, but it has a small problem.

The rear sight walks over to the right after just a couple magazines. I tapped it back in place with a rubber mallet. Is there a set screw or anything in there or is it just friction fit? How do I make it stay put?

Thought I would get advice before I start tinkering with it.

Take it to Bert at BP Tactical, he'll fix you up cheap. He'll he might even trade gun smithing for premium brand cat food, just ask......
His contact information is in the Industry Partners section...