It's where the fuel pressure regulator was for FI, just cover it with anything, it's in the front tinware not firewall.
sorry i meant a picture of that bit. i knew it wasnt for fuel reg. but couldnt think of what that bit is called to be able to google it.
Your bus was originally FI as I have the original engine from it, and got most of the FI stuff and tinware with it too, including the original one of those tins! So that one could have come from anywhere! And as others have said that hole was for a carb/air pre-heat feedback loop coming from a similar hole in the tin by the heat exchanger.
The engine tin is different for carb and fuel injection. Rob, you have carb tin and as dicky said it's for the warm air supply to the carb airbox on a stock twin carb set up. The fuel injection engine doesn't have this large rectangular hole but a smaller round one in a similar place for the fuel pressure regulator. I only know because I had the wrong tin on my engine!