I'll never be dissatisfied with any amount of effort given. All I can ever be is appreciative of what has been done! Thanks again!
This building probably started off being called the Aluminum Smelter and used the text key for that but then was later changed and rather than eliminating the old text key, a new one was added. We should probably at least comment out the old text key reference in the XML.
Probably same as above except that when the doc was initiated, we had an Artist Guild and since then it's been renamed Artist Colony and the last person updating the list didn't remove the Artist Guild from that list either. At the point where we've gone through all of the XML building documents, we will know for sure if the Artist Guild entry in the spreadsheet is to be removed, so it can be safely left there as you did for now.
There's two options as to where the TXT KEY could be referring to then. One is the Modules folder... I assume you probably included that in your grep search. The other is one of the foundational core TXT files.
If the system cannot find any xml type referred to, including text keys, in the mod files, it will then check the Beyond the Sword xml files. If it cannot find it there, it will then check the Warlords (if it exists) xml files. If it cannot find it there, it will then check the Civilization IV xml files. Only if it fails to find the reference in all of those will the loading of the XML that refers to the missing type reference deliver an error (usually). TXT_KEY references won't give any dramatic errors... you'll just see them displayed as exactly the text key reference as written if it couldn't find a text to match to it, aka, the building name in the pedia would thus just say TXT_KEY_BUILDING_WHATEVER.
I suspect that this TXT_KEY_BUILDING_GERMAN_ASSEMBLY_PLANT was probably a Unique Building from Vanilla BeyondtheSword or even as far back as Civilization IV unexpanded at all. Check those text files. I have a feeling you'll be able to find them pretty easily now that you know they are there.