help..what is hole between oil pump and oil filler neck?

[katykoo]

2litre a/c-Have sourced the reason for my bad oil leak, it appears in a small, 10p sized recess in crank case to the right of the oil pump and before the filler neck.It looks as though a bung should be there..any help or ideas appreciated. thanks

[katykoo]

have high lighted the area concerned.

[katykoo]

I attach a photo that appears to show aflat surface-bung where the hole is..