Have pulled the engine and rear end out of my car. Inside the bell housing of the gearbox was brass shavings from the starter motor bush. Also there is a groove been ground into the gearbox has been rubbing on the outside edge of the flywheel. What would likely cause this?

was the engine to gearbox bolts loose?

or perhaps someone fitted a 200mm flywheel into what was originally a 6V 180mm flywheel gearbox?

The bolts were all tight but one was missing. Gearbox has 113 301 103 on it with 40 and C as well.

Maybe rear main bearing has excessive clearance. Like heaps excessive .
Is the engine in generally excellent, good, fair or poor condition?

Could be a combination of all of those. Sounds like a 12v flywheel in a 6v gearbox, a solid brass starter bush sounds like a conversion bush. The gearbox case may have been clearanced properly but the starter bush has failed and pushed up a burr on the gear teeth that may have cut a new “track” that you can now see. File the burrs off the flywheel before you put it back in, after replacing the starter bush, and checking the rear main bearing’s fit in the case. While it’s out you’ll be putting a new silicone rear seal in of course?
I’ve also seen a clutch pressure plate break a retainer strip, it swing out, and cut the bell housing off. You would see that though.
Good luck, Tony.

motor is in good/fair condition I guess. This is my first attempt at working on a vw engine. How would I check the main bearing clearance. I really don't to have to pull the case apart). It looks like the starter bush has a ctually been chewed by the flywheel. The bellhousing looks like it has been attacked by a grinder by a previous owner. Possibly to fit a 200ml flywheel.