That hole in the side of the block is a kind of emergency exit.
When the main seal (directly underneath the impellor) starts to leak the coolant will take the path of least resistance,
the slot underneath the pump. Thus preventing coolant mixing with the engine oil.
The O-rings you mention are only secondary seals, there to prevent oil or coolant leaking between brass cage and block.
The main seals sit inside this bass cage.
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<font color="blue"><i>1976 TR7 FHC (needs some TLC ...)
1980 TR7 DHC (my first car, a.k.a. Kermette)
1981 TR7 FHC (Sprint engined a.k.a. 't Kreng
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