I have owned, serviced & repaired many Sierras ( Both Own cars & customer ones )
I have never seen a MT75 box in anything other than the 2 Litre DOHC engined Sierra.
I do know that the last hadfull of petrol Sierras had either the 2L DOHC engine or a revised 1.6 CVH engine
These changes took place in late 91
Perhaps the dates given on the Zetec inside forum refer to the dates that the earlier, 4 speed, type 9 gearbox was last used.
Odds on, if you find a donor that is not a DOHC model, it'll have a type 9 gearbox.
I hope this is of some help
My first donor was a 1991 1.8 CVH estate & was fitted with a type 9, 5 speed gearbox