Can confirm exactly as the S pivot. The S got ruined by the later Challenger incarnation employing the Maxi grip rail and raised trigger block.
One of the main features of the Mercurys/ Airsporters (forever) was the graceful trigger block.
If they had wanted it made taller to act as an arrestor behind the rail, why didn't they just opt for a stud in the Maxi rail. It would have saved costs by not having to have new tooling for casting the taller block.
The Challenger stock was better however, with traditional higher quality chequering.