Can someone please tell me what is the purpose of it?

Is it small, on the china wall, just to the driver side of the distributor?

If so, some oil pressure sending units mounted there in the old days - it may just be a plug for that hole - I have my OPSU mounted there now.

Are you asking about the bolt holes on the rear ledge, adjacent to the casting number? (Round head slotted screw in this one):

Yes, Camaronewbie, thats the one near the distributor on the china wall I have it on the present engine and the original one.

It may well be the old location for the OPSU then. See the pic from Vader above - there is a brass plug with a square head and white sealant around it just above the oil filter - this is where most OPSU's are at these days - but in the old days, OPSU's were often in the china wall close to the distributor - many use this small hole for their mechanical oil pressure guages - they tap in the connector for the little plastic tube that runs the oil into the dash for the mechanical oil guages.

Thanks!