FX Crown Mk2 transfer port

My new FX Crown Mk2 (.25X600) has two transfer ports on the barrel.

One side is just a conventional hole with a "P" marking. If you rotate the barrel 180 degrees, a dual port marked "S" is exposed. 

Yes, it has indexing for both ports, so that you can choose. "P" for pellet, and "S" for slug? Don't know.

Anybody know what this is about?

EDIT- Barrel came from factory with just the single "P" hole as the air source path. Dual port was at the top.