The circa 62 engine would have had an alternator, so about everything is mounted and fitted different.
The hole to the crankshafts left (our right) was to mount the inner tin chaincase - there should be a bolt there, holding it on.
You may note that some PO has cut the (black) tinwork away, so the hole in the tin to suit has been removed.
I could show a similar inner chaincase off my ES2, but its being reworked and rewelded to repair that area.
The half moon cutaway and oil leaks don't sound standard either.
There are usually pen steel washers inside the crankcase, and possibly some felt seals, to keep the oil in.
I'm a bit fuzzy on this, been a while - a study of the parts book and diagrams may bring it back.
If your engine is wet sumping (oil leaking through the oil pump into the crankcase) while standing,
it may well get enough to leak through there while on the sidestand.
Have you drained the oil in the sump when it does this, to see how much comes out ?
It should only be a half cupful or so.
If its more, and the oil level in the oil tank is also dropping, then the oil is not staying in the oil tank,
but escaping through the oil pump into the sump, and then out around the crankshaft there.
Measure the sump contents, and we'll go from there.
I didn't even know they made the 500t as late as 1958.
Maybe we could see a pic of the whole bike sometime, be interesting.