I think this thread could benefit from some acknowledgement of the actual people who made those Head, PDP, Prince, Adidas, Le Coq Sportif aluminum frames through the better part of the '70s.
The similarities between the butt-cap stickers of these racquets are impossible to miss. They all list the same three patent numbers.
The inventers responsible for these patents are George Vaughn and Richard Hargrave of Maark Corporation, located just outside the city of Princeton. The racquet illustrated in the patent drawings looks very much like the PDP Open/Adidas 660 (which differ from one another in a couple of minor but non-trivial details). Vaughn and Hargrave also held a design patent for the throat piece on the original Prince "Classic" (as a result, the only patent out of the five listed on the Prince butt-cap that didn't come from these two men was the infamous 3,999,756 from Howard Head).
I've read here and elsewhere that early Head and Prince aluminum racquets were made in Colorado, but I wasn't able to find any reference of a Maark manufacturing facility outside of New Jersey. In fact, Maark Corp was so low-key that there was precious little written about it anywhere. The only solid piece of information I found was this short article in the Princeton alumni newsletter (Vaughn and Hargrave were classmates). The article focuses on another alumnus who was in charge of Prince Manufacturing at the time, but the section I highlighted is the most interesting part, I think.
As one can see, Maark Corporation was the 'Kunnan Industry' of Princeton for a number of years, until Head bought them. Subsequent to the acquisition, Vaughn became the chairman of AMF Head Racquet Sports. By then, graphite technology was already nipping at the heels of the aluminum juggernaut, and 'Made in USA' tennis racquets were rushing inexorably towards the endangered species list...
In retirement, George Vaughn continues to sit on the board of trustees at Vaughn College of Aeronautics and Technology, a school co-founded by his namesake father, also a Princeton alumnus, who was an ace fighter pilot during WW I (I wonder what he thought of his son's inventions
As for PDP, the only short-lived brand out of the foursome above, I had a hunch that it was to Maark Corp what Pro Kennex was to Kunnan Industries, purely on the count of the close resemblance between the Open and the line drawings in Vaughn and Hargrave's patent applications. However, available evidence does not support this hypothesis, as PDP was apparently acquired by Le Coq Sportif in late 1980.
So Maark Corp probably never bothered to build a house brand (until they were absorbed by Head). They were perfectly happy to toil away in complete obscurity, allowing their OEM clients to bask unfettered in the glow of their technological success. There are many advantages to this business model, but the most obvious downside is that you don't get proper credit for what you've been able to accomplish.
Those Princetonians must not have suffered from insecurity issues much.