I was under the impression that the IVs/Vs and the reintroduced TBird had the same pups initially. Then the IV/V fell by the wayside and the TBird carried on getting eventually vamped up versions of the TB pup. To all intents and purposes the IV/V should be a modern ergonomic TBird at its outset (Phil Jones, who designed it, told me) it wouldn't have made sense to have consciously given it different pupps to what it should emulate.
It wouldn't have made sense (when did that ever stop Gibson?) and you may be right, and Carlo may well be right that this could be a transitional pickup. OTOH it is different and the IV wasn't advertised as having Thunderbird or TB Plus pickups. So I don't think we can be sure unless you contact Phil and he can confirm that this particular variation was also used in some late 80s T-birds.
It wouldn't be the only time Gibson used a similar-looking but different pickup. They did that with the SG-Z.
