Gallery plug behind oil filter housing?

[Hugo]

I am still trying to solve the riddle of my oil gallery plugs. I have found a home for the extra plug - it was actually a water plug on the opposite side of the block.
But what I am still trying to establish is whether there should be a blanking plug behind the oil filter housing on a late Series II. I have the plug, but I cant see any point in fitting it, as there is an adjacent hole through to the gallery which communicates with the same chamber in the oil filter housing.
What is the point of fitting a blanking plug that doesn't actually blank anything off?

[christopher storey]

You need to be a bit careful about this, Hugo . There are several different filter/filter head arrangements which have been used, and which use different gaskets, and if you mix and match the wrong gasket with your head it can have serious effects . This problem sounds to me to be analogous to that. For what it's worth, my S1 4.2 parts list shows 5 plugs for the oil gallery and 1 adaptor for the ignition warning light/oil pressure switch i.e 6 holes in the gallery to be filled, and I am pretty sure the S2 block will be the same

PS I've just looked at the S2 parts list , and this shows

6 X C 2352 side gallery plugs

1 X C 2157 rear oil gallery plug

I front gallery brass plug

AND ALSO 1 X C 15867 Oil Filter Flange Plug

Could this last have something to do with it ?

[Hugo]

Yes, that's the fellow. My gasket set had no fewer than five gaskets for the oil filter housing - all different. I just matched the gasket to the housing. I suspect that this plug is intended for a different filter head, where this hole needs to be blocked off. But on mine, there is an adjacent hole (in fact, I would guess they all have a hole next to where this plug lives) which communicates with the same chamber on the filter head, so it seems the only thing this plug would ever do is fall into the oil gallery if it comes loose.
I'm going to have to pop the filter head off again to double check this, but I'm quite sure I'm right.
Perhaps they just fit the plug to all engines, regardless of the oil filter head that is actually fitted? Other than that it seems to be rather like the human appendix, or men's nipples, which don't do a lot.