Remember that the parts list is from 1937, I did see the 1938 Big 4 had a fixed connection too, and another 16H aswell, but believe later ones had the adjustable type.
More research needed! to make the fixed type seems useless, unless you have a pre 1940 Norton bike. (I'll have a couple though Ron! they seem to be pressings instead of castings?)
If it's original to an India Office bike rather than to a retro-fitted tank top filter then it is likely to relate to one of the rear-mounted cannister filters.