The photo of the underside of the action shows the button only on the left side of the trigger guard. If it were a selector it would need a protrusion on the right side too, otherwise how would it switch from left to right and back again?

I am betting that it is a release catch along the Gamba lines.

The V sprung action does not seem to incorporate rebounding hammers (tumblers).

At 3.64 kilos it outweighs a game MX8.