it's where the ignition pick up wires would come out on a kokusan ignition carb 900ss, which shared the same cases. maybe? maybe not exactly the same cases, not sure if the swingarm pivot is actually the same for 907 and 900. parts catalogue says not.
but i guess they ran most machining steps on all cases. st4s still had kokusan ignition pick up bosses with threaded drillings from memory, which no 996 stud spacing cases ever used. i guess with automation it's no big deal, but it's still redundant steps in the process.