There are two variants of keycap scissor clip design commonly used on the pre-unibody MacBook range and two on the white Unibody range.

In almost all cases MacBooks prior to the late 2007 (Santa Rosa) models use type E keys.
Late 07 right through to the last mid 2009 models of the original style MacBook use either the type E or type F keys. The white unibody MacBooks introduced late 2009 use either type E or type G keys.

Where there is a choice you need to check the clip design of your keyboard against the illustrations below to confirm the type. The key type identifies the type of the whole keyboard i.e. if one key is identified as type E, the whole keyboard is type E.

All key caps come complete with their scissor clip.