Halliday & Matthiessen (2014: 404-5):
System II is that which is available after a past projection [i.e. projecting clause] such as they said. … What happens here is that in the environment of a ‘past’ feature, the past element in three of the System II tenses is neutralised; past, past in present and past in past [are] all represented as past in past. Since there are six such triads, System II has 2 x 6 =12 fewer tenses than System I.