Aha.
I'm afraid I can only guess about the history / configuration of your specific engine. I suppose it may once have housed a Reliant Crank but if so (and if it was done properly) I'd expect to see other things going on too.
What I suspect may be the case here is that someone has drilled down through the main oil gallery so as to link it with the hole for the long offside centre main bearing stud. From the other side they would have removed the screw which secures the centre camshaft bush, and via this hole extended the drilling of the cam bush oil supply across to link up with the aforementioned stud hole. With this done, and perhaps some modifications on the long centre bearing studs & holes, you then have an oil supply direct from the main gallery into the top of the centre main, which is probably better than the original Austin arrangement (which was prone to fractured pipes etc. as well as supplying oil to the "wrong" side of the bearing).
On other engines I've seen the cam bush oil supply is then blanked off; the bush is instead drip fed via a small hole drilled in the floor of the block from the valve chest (which is perfectly adequate).
If all the above is correct, you have no need to worry that your centre main is getting no oil. However as with all modified engines, you really need to have it apart and explore to understand in detail what has been done to it. Meanwhile I would advise you to take very great care if replacing any engine parts in case you upset the workings of the present arrangement. For example if your cam bush is drip fed, replacing the block with another could cut off its oil supply, should you fail to spot the additional drilled hole.
p.s. yes I know I'm a bit tight but frugality used to go hand in hand with owning an A7...
I'm afraid I can only guess about the history / configuration of your specific engine. I suppose it may once have housed a Reliant Crank but if so (and if it was done properly) I'd expect to see other things going on too.
What I suspect may be the case here is that someone has drilled down through the main oil gallery so as to link it with the hole for the long offside centre main bearing stud. From the other side they would have removed the screw which secures the centre camshaft bush, and via this hole extended the drilling of the cam bush oil supply across to link up with the aforementioned stud hole. With this done, and perhaps some modifications on the long centre bearing studs & holes, you then have an oil supply direct from the main gallery into the top of the centre main, which is probably better than the original Austin arrangement (which was prone to fractured pipes etc. as well as supplying oil to the "wrong" side of the bearing).
On other engines I've seen the cam bush oil supply is then blanked off; the bush is instead drip fed via a small hole drilled in the floor of the block from the valve chest (which is perfectly adequate).
If all the above is correct, you have no need to worry that your centre main is getting no oil. However as with all modified engines, you really need to have it apart and explore to understand in detail what has been done to it. Meanwhile I would advise you to take very great care if replacing any engine parts in case you upset the workings of the present arrangement. For example if your cam bush is drip fed, replacing the block with another could cut off its oil supply, should you fail to spot the additional drilled hole.
p.s. yes I know I'm a bit tight but frugality used to go hand in hand with owning an A7...