Only one type of Box for the ST170 and it is a getrag MT285, there are two types of clutches though, early models like yours had a smaller clutch plate, later ones had a larger clutch plate, the smaller ones tend to slip. There is a metal blanking plug on the front of the box, park the car on level ground, undo the plug and the fluid should be just up to the lip of the hole. Best thing to do is buy some new fluid and squeeze it into the hold until it starts running out, then you know its at the right level