Now, for most of my D&D years, I assumed this describes a way to trade points to get higher scores. I probably believed this because, when I first learned original D&D, I didn't see the books and we didn't use the p0int trade. The first time I saw these rules was in Holmes, and there they seem to be presented as a point trade. So, when I ran a group and we acquired AD&D and the LBBs, which we mixed, we used point trading to improve rolled attributes.
Strength is the prime requisite for fighters. Clerics can use strength on a 3 for 1 basis in their prime requisite area (wisdom), for purposes of gaining experience only. Strength will also aid in opening traps and so on.
Intelligence is the prime requisite for magical types. Both fighters and Clerics can use it in their prime requisite areas (strength and wisdom respectively) on a 2 for 1 basis. Intelligence will also affect referees' decisions as to whether or not certain action would be taken, and it allows additional languages to be spoken.
Wisdom is the prime requisite for Clerics. It may be used on a 3 for 1 basis by fighters, and on a 2 for 1 basis by Magic-Users, in their respective prime requisite areas. Wisdom rating will act much as does that for intelligence.
Now, though, looking at that phrase "for purposes of gaining experience only" -- which I've heard people mention recently, but didn't pay attention to -- I realize this has nothing to do with making clerics wiser by lowering Strength. The scores aren't being changed at all. What the rules are saying is that clerics gain more experience if their Wisdom is high, but Intelligence and Strength contribute to earning experience, to a lesser extent. In contrast, Fighters gain more experience if their Strength is high, but Intelligence and Wisdom contribute to a lesser extent; MUs likewise gain more experience if their Intelligence is high and Wisdom contributes to a lesser extent, but Strength doesn't play a role in their advancement at all.
Now, the way I interpret the attributes is completely different, so the question of whether this was a point swap or not is moot. Still, it's interesting how the original meaning of the 2-for-1 and 3-for-1 equivalences was lost, and quickly.