You have a good eye I have never noticed the small round bracket on the chassis before, the small hole on the tank upper bracket is to secure the mounting bolt. Perhaps the bracket on the chassis was for a chain to hold the cap?
I just saw this one on Ebay, nothing to do with me so check before you buy.
Yes "thread drift" can be an issue on this forum provably because most of us just tend to reply to the last entry on the thread rather than going back to the top of the thread, I know I've done it myself, or it could be the title of the thread, original monocoque construction instead of looking...
An EBay search will return a couple guys selling them, I don’t think anyone is selling the complete set of drawings but it may get you started.
Nothing to do with me but I figure the more the merrier, more cooperation between the people building them, I hope.
Search for gt40 blueprints
This may help.