They are both 6 lens taillights and the screws seem to be in the same placement. But there is a distinction mentioned on all parts for the 240 Volvo that separates 1986-1993 and 1980-1985.
I do know the newer taillights have a hole for adding a fog light which I don't have or plan on getting. There is also quite a difference between the electrical system from my old 1982 Volvo 240 and my 1990 Volvo 240.
If the lens assembly is the same but not the circuit, I could replace my lights and circuit board into the 1984 taillight as it is just my lens that is broken. Lights and circuit work fine.
Anyone know if the 1984 6 lens taillight lens assembly is identical to the 1990?

give it a shot and see. That's what I would do.