I've done some homework on failures of this type of chainplate arrangement. Fortunately, they are few but all of them have been at the thread section and usually with evidence of moisture intrusion. The liner is cut away so that I can do a cursory inspection and although there's no evidence of moisture, corrosion or rust I know it's where I CAN'T see that could be a problem.

I'll contact the manufacturer. I'm quite sure these pieces aren't available through them, but as you say they may be able to recommend some good shops.

If I do have new ones built I'll at least be able to take the old ones in for duplication which I know shops appreciate.