[GVS]

To build on Joyridin's comment, Arntz frame (which was made of rectangular tubing) were an evolution over time. They are generally categorized into 4 buckets: Types 1 thru 4. Since the cars were constantly evolving, these buckets are defined by the following significant changes:

• Type 1 were the first of the bunch: the body finish was not as good as later cars and the chassis lacked the “safety cage” around the passenger compartment. These were introduced in 1972.
• Type 2 had a nicer body finish, a “complete” chassis with formed steel tube safety cage around the passenger compartment. According to Jason Smith (former Butler employee), Steve Arntz added these safety features after his girlfriend was killed in an accident while in an Arntz Type 1.
• Type 3 has more body refinements including nicer body with 'drip rails' at the hood and trunk openings for attaching real weather stripping, nicer foot boxes, improved dash and wiring. Arntz went out of business at this point (1980) and sold the design, body molds, etc. to Butler. Butler continued to improve the Type 3 design as he produced his cars (1980-1998). Butler built a total of 104 vehicles.
• Type 4: I have read that Steve Arntz was a constant tinkerer. He still had ideas for how to improve his Cobra design and created a Type 4, more as a hobby. 16 of these vehicles were eventually sold as kits; 5 are known to be complete and on the road (according to Jason Smith). I do not have any other details on this vehicle.

I hope this helps!