From a press release issued by the FIM:

Superbike World Championship Regulations – Decisions of the Superbike Commission

The Superbike Commission, composed of Messrs. Claude Danis (FIM), Paolo Alberto Flammini (SBK International), Takanao Tsubouchi (MSMA) and Giulio Bardi (Team representative), has unanimously decided to introduce the following modifications to the Road Racing World Championship Regulations for Superbike (application as from 1.1.2003 only for 4-cylinder 1000cc machines):

Art. 2.4.4.1 Restrictors
1) All the air feeding the engine must pass through the air restrictor(s).
2) If the air restrictor is NOT included in the throttle body as homologated by the FIM, the air restrictor must be located somewhere between the throttle valve at the throttle body and the cylinder head (not including the cylinder head itself). The air restrictor must have a round aperture with a diameter no larger than 50 mm for 2-cylinder engines (as from 2004) and 32.5 mm for 4-cylinder engines and must be of non-deforming metal or metal alloy with a thickness of at least 3 mm (It is necessary to check its diameter on two (or more) directions).

3) If the air restrictor is included in the throttle body as homologated by the FIM, the shape of the restrictor may be oval, but the area of the aperture must be precisely the same as for a round restrictor, as mentioned above.

NB. This modification also applies for the Endurance World Championship