Oil seal 'o' ring?

[jmw]

I took the flywheel and main bearing housing off my S1100 the other week.
I noted how the oil flows from the pump into the main bearing and out the other side where it takes that external pipe route to the top end.
I also noticed that the oil flow hole in the in-coming side of the bearing housing has a small circular recess in it, which looks like it is perfect for a rubber 'o' ring type seal. However, I could not find any 'o' ring. The plant room is very dark and the floor is dirty and oily, so I don't know whether I have lost the 'o' ring, or it never existed. The engine manual does not show an 'o' ring, so I am hoping that I don't need one for re-assembly, but if this is true, what is the purpose of the recess?
Does anybody have any knowledge or ideas on this?

Thanks,

    Mark