I'm going to throw my 2 cents (sorry 5 cents now) in & say it could be an early 4 speed box from a 1000 coupe. My friend had one years ago & it was definitely a 4 speed shifted through the top of the transmission tunnel. It'd make sense that as the capacity/torque of the engines increased from 1 litre up to 1.2 & beyond, they'd put strengthening ribs on the casting to stop the case twisting. The hole the clutch cable goes through is definitely the earlier 1200 size, not the later 120Y/Sunny size.