It used to be the case that 74I represented an OF mid-J, and 74J meant a NF mid-J. That's the information that's incorporated into the internal codes thread (link in my signature).

We have, however, been getting quite a lot of reports of flights that have said 74I turning out to be NF aircraft after all.

What isn't clear is whether this is because the codes have changed (in the sense that they now represent different things), or because there is simply an extension of the phenomenon that CIHY reported on a long time ago, namely that aircraft swaps between 74I and 74J were so common, and often so late, that there is no point trying to work out in advance whether your flight will have OF or NF until you reach the aircraft and see whether zone A has the NF window blanks.

What I have seen myself is that 74I is used both for "OF sold as J" flights and also for conventional 4-class flights. But I haven't yet been trawling to try to see whether 74J is still being used as a code.