This parts covers the flywheel and clutch located between engine and gearbox. It is also used to mount the starter motor which makes a connection to the ring gear on the fly wheel.
The original bell housing was corroded badly and one of the previous owners has made a square hole in the bottom. Most probably to inspect the clutch or do some kind of repair without removing it from the engine. I found a nice replacement in the UK as can be seen in the next picture.
In the next picture the assembled gearbox has been mounted onto the bell housing. The clutch fork and clutch bearing can bee seen on the photo in more detail. The next step is assembling the top cover of the gear box. When this part is finished everything can be bolted, together with the clutch and pressure plate, to the engine. The protection plate must be mounted when the engine and gearbox are bolted together.
Because the flywheel, pressure and clutch plate can’t enter the bell housing when the cover plate is mounted already, it will be done after it has been bolted to the engine block. This cover plate prevents that water, crease and dirt enters the bell housing and will have a bad influence on the clutch operation.