We have one previous thread on that engine Mark:
https://www.outdoorking-forum.com.au/forum/u...p;Words=yamaha&Search=true#Post11309

It does answer one of your questions: it seems the engine has a specific oil reservoir for the bottom crankshaft bearing.

I don't think anyone would make an engine that had a brass gasket in contact with an aluminium engine: the galvanic cell thus created would result in corrosion at awesome speed, with the aluminium part being the sacrificial anode. Most likely it is some kind of coloured anodized finish on an aluminium part.