Gradius 3 (Japan arcade) - which loop does difficulty max?

[meco1999]

Does anyone know which loop difficulty maxes out on on Gradius 3 (Japan arcade)? I checked it using MAME cheats, and at first I thought difficulty stopped increasing with loop 4, but looking more closely it seems to maybe stop getting harder at loop 10. Does anyone know? I'm just curious really.

Also I wonder if most of the Gradius 3 arcade machines were set on the Easy or Normal dipswitch when it was in arcades?