Pajo,

The rectangular hole in the piston is to prevent masking of the boost port, when the piston goes down.

It goes on the piston side, opposite the exhaust.

The Schneurle ports are angled on both sides of the exhaust ports and the third "boost" port, usually with slightly shorter open timing, is angled up, opposite the exhaust post.

The ports are all angled, so as not to spew the fresh mixture out of the open exhaust port.

Don't get the carburettor spray on plastic and neoprene parts.

Use after-run oil, or ATF (or Marvel air-tool oil) for assembly.