My guess is that it's not an external flange at all, but an aerodynamic station in the main core flowpath. For example - depending on the vintage of the engine and the industry ID standard at that time - A4 could be the flow area of the stage one turbine nozzle. An A4 flange could be shorthand for the nozzle flange that (internally) holds it in place, and a seal at that location prevents (or reduces) parasitic air leakage.
But it's just a semi-educated guess.
