After extensive research it turns out to be a 1970 1/2" Rear Sway Bar, Part number D0ZZ-5A772-A, used on Mach1 and some Boss 302's
Some 70 Boss 302 cars left the factory with 1/2" bars and others with 5/8" bars.
Also some 428 Mach1 owners report having this 1/2" bar from the factory.
Standard on 70 351 Mach1's