I was interested to read your post and especially in reference to the outsourcing of castings.
I remember seeing castings for Stanley plane bases/soles when I worked at George Fischer Castings in Bedford during the late 1980s.
It was my understanding that the parts were cast and fettled on-site...
You could always give these guys a try
Pro Machine Tools Limited
17 Station Road Business Park
Barnack
Stamford
Lincolnshire
PE9 3DW
England
44 (0) 1780 740956
I've bought some parts and accessories for my Unimat 4 from them and the guy there is very knowledgeable.