I have a copy of the '1974 Dirt Bike Buyer's Guide' in front of me, which covers all the bikes and aftermarket goodies available in 1974. In the 'Frames & Swingarms' section, there aren't any DG products in that listing. However, you could buy a square or rectangular section swingarm, in 1974, from Red Line, Cycle Products West, Alex Steel Products and Ocelot. Those four manufacturers all display square and rectangular swingarms, both stock length and extended, for Honda, Yamaha, Kawasaki, Suzuki, Bultaco, Jawa and Hodaka. Or, companies such as Cycle Products West, who make rectangular swingarms will "also build a custom swingarm for any bike for $22.95."
It would seem to me that anyone today could legally have a square or rectangular swingarm on any Pre '75 bike.