There is a "main" MK IV frame but some differences and modifications for standard and lightweight cars are existing, for example safety belt attachment, the pedals, the roll bar bar attachment points, lower front wishbones, the doors, door lock attachment on frame, transmission mounting points and so on.
The complete sub frame behind the seats is different for the lightweights and more MK III (US definition for the MK) like.
I'm not talking about the pre MK IV frames.
I will add some MK IV frame pics in the evening
1st top: MK IV standard frame, transmission mount does not have the ears on the side of the main tubes for Ford Mustang like transmission crossmember in rubber bushings as the 5.0 carburetor cars. Transmission attachment is at the rear.
2nd: Autokraft plant, front is MK IV Lightweight frame behind is MKIV standard
3rd to bottom: MK IV Lightweight frame details
