N32 already goes up to 256 bit, so it doesn't make much sense to use N31 with 4 MCDs when they can use N32 with 4 MCDs.
I foresee the full N32 going into the 7800 XT and the 7700 XT being a cut down version with 3 MCDs/192 bit.
A full N32 only has 64 CUs and 64 RT cores. A cut down N31 with 74 CU and 74 RT cores even with the same 256bit memory will be faster.
So far we know they will only make 3 Chips, N31, N32 at 5nm and N33 at 6nm. N33 at 6nm will not be much faster than current N23 (RX6650XT) thus will not be able to compete against AD106 RTX4060.
For that reason they need to make 3 SKUs per Chip if the want to have enough SKUs to compete against all NVIDIA AD cards.
NAVI 33 at 6nm = 204mm2 and 13B transistors
4nm has almost double the density and AD 106 is 190mm2 (~20-22B transistors), there is no way that N33 (7600XT) will be able to compete against that chip.
So if N33 will not compete against AD106 RTX4060, then it will need to compete against AD107 RTX4050. Meaning there will be a huge gap from RX7900XT to 7600XT that they will have to cover with only a single NAVI32 chip.
Either they have another Chip or they have to create 4 SKUs from a single NAVI32 chip (RX7800/XT and 7700/XT), not possible. Well now that I think about it, they may as well create only two SKUs, 7800XT and 7700XT.
But again, a cut down version of N32 RX7700XT will only compete up to 4600/Ti and 7800XT up to RTX4070.
This is the reason a even more cut down N31 for a RX7800XT card will make sense and help them have a card between RTX4070 and RTX4070Ti, leaving the N32 for three more SKUs (7800/7700XT and 7700).