The definition of NT is found in D1.33 - "Any MA of the "NT" type is recognizable by having no MA Type white circle/square on the counter."
Rule D1.3 above it states that the T/ST/NT etc. restrictions apply only to the Main Armament. "The MA is either turret mounted....or bow mounted (i.e. non-turreted)." The BAZ in the M20 isn't bow mounted (if it was, it would be printed on the counter).
Case 3, C5.3 says "Vehicular (including Passenger) ordnance which fires during its MPH must use one of the Case C DRM (i.e. it must add the +2 DRM of Case B to the applicable Gun Type DRM of Case C)...."
This is saying the T, ST and NT DRM are "Gun Type" DRM. However, a BAZ isn't a Gun (per the Index definition), it is a SW. I would conclude the "NT" restriction, referred to as "Gun Type DRM", only applies to Guns and/or MA. There is no "applicable Gun Type DRM" since the BAZ isn't a Gun.
The Vehicle Note for the M20 lists the AAMG as the MA, and the BAZ only exists via US Multi-Applicable Vehicle Note Z. It isn't the MA, it isn't a Gun, and it isn't subject to Gun Type DRM (which is what the NT restriction is).