The Yamaha "YCR216" is a foam-cored aluminum sandwich design, adapted from contemporary ski architecture, just like the Head "Arthur Ashe".
At the time, the term "composite", as used in the context of tennis racquet classification, simply meant that two or more dissimilar materials were combined to create the frame in question. Early fiberglass- and carbon-fiber-reinforced plastic frames were given their own categories for a few years, in part due to their novelty, but also because they were qualitatively different from the metal+plastic, metal+wood, and wood+plastic composites. It wasn't until the market became irreversibly dominated by "graphite" frames that "composite" acquired its near-exclusive association with fiber-reinforced-plastics.
The FRP racquet era is only half-a-century old, younger than many of the forum participants here, so information on racquets of this type is still readily accessible relative to that of the truly ancient stuff. As you've undoubtedly noticed by now, a great majority of the archived discussions on this sub-forum revolve around this category of frames.
Since you tagged me on the "WB-210" and the "XT", I would just add that both of these racquets took far longer to reach the market than one might have expected.
Warren Bosworth claimed during an interview that he had designed the "WB-210" in 1974, and the companion "WB-215" even earlier - in 1971! Given that he was not a trained engineer, had no manufacturing experience or resources, and was just learning to string and customize racquets at the time (and quickly became so good at it that he quit his day job to turn it into a very successful full-time business), we can only surmise that those original designs went no further than sketches on a piece of paper. Bosworth's relationship with Fox started rather late but was extensive. It was similar in nature to the one he had with Snauwaert - he got to rely on their engineering and manufacturing expertise to turn his drawings into actual products; in return, they got to use his name and fame to sell more racquets. Win-win! Both the "WB-210" and "WB-215" came out in 1985. The angular shape of the former (which offers no known advantage over traditional ovals, at least none that anyone could explain/justify using physics and/or empirical evidence) lives on to this day in the form of a made-to-order racquet offered by Bosworthtennis.
In contrast, the design for the "XT" was already fully worked out by the late 1960s. Circumstantial evidence suggests it was intended to be an OEM for Head. If the project had gone thru, Head would have had an indisputable claim to the title of the first all-synthetic racquet in the world, as the "XT" would have beaten the efforts from Völkl, PDP, Montana, etc. by up to three years, in both IP and development! However, many in the industry saw plastic racquets as a passing fad at the time, and Head was fully committed to building and marketing their in-house aluminum sandwich design, based on the ski architecture that made their founder famous. As a result, the "XT" was not born until 1975, lagging well behind its competitors, and, compounding its misfortune, it was instantly eclipsed by the first wave of "real" graphite frames launched that same year, including one by DuraFiber themselves. Meanwhile, Head did not have a plastic racquet of their own until late '76 - the 100% fiberglass "XRC", which they claimed was superior to graphites in their advertisement, echoing the foresight that may have led them to pass up on the "XT" many years earlier.
On a completely unrelated note: Is anyone else having trouble accessing talk tennis through their home wifi? I have not been able to connect to the forum using wifi since last night, but it works perfectly fine with my phone. Apparently it's a DNS problem having to do with where these discussion boards are hosted, as other parts of the TW website are not affected. I used live-chat to try to get some help from TW, but their help agents don't have any problem connecting to these boards themselves, so they have no idea what is going on. Surely I'm not the only one having this issue at the moment?
