Perhaps a surface casting defect as the outer part in the mould cools the fastest. Castings are a bugger to inspect, as had the same problem years ago inspecting some rope sockets.
I am a bit cautious about welding on castings as due to their inhomgenous nature they can react badly and plus your ones are quite old. Given that he is saying no pressure what about silver solder, or would that be to much heat input?
If silver solder is a no go then a dab of devcon on the outside could stop the weepage.
One last comment. Regarding cracks, due to the stress concentrated at their tips, they almost always grow. In basic terms, if you have a bar with a 1 ton load on it and drill a perfectly reamed hole in it you create a stress raiser which multplies the stress by a factor of 6.
If said hole is nicked or cracked the stress can be increased by over a million, hence cracks tend to grow and a method of repairing cracks is drilling a hole at the ends.
Anyway, glad you have had good news as is good to be able to move on....... When would you expect it to be running as you have most the bits now? Or are you missing some bits?