Consider SLS 3D printing. It requires a tiny amount of sanding and filler primer. Far less than FDM parts. Also SLS parts also don't suffer from overhang issues, so you get increased accuracy in every dimension.
I find that SLA parts are very accurate, but the resin gets sticky over time, Good if your using it just to make molds, but not for moving parts.
Machined aluminum is obviously the best. (looking at you Mr. Savage)
Considering the third hole is counter-sunk, it makes sense that it might be for a third screw. It just may not have been present when the reference photo was taken.
As I posted above, these covers typically have a post with a radius cut into the bottom face, in order to complete the bearing race where the feed hole meets the race. It is odd there appears to be a hole here though. Doubtful it's a lubrication hole, because in addition to oil, dirt can get in.