I am a little surprised that this model appears to have flown under the radar somehow. My '95 edition Kuebler gives the impression that Yamaha's YFG model numbering system was at least partially based on head sizes (e.g., YFG-20= 20% more face area than a standard head), and that in addition to all the model numbers that ended in "0", they also produced a few frames with model numbers that ended in "5" (i.e., YFG-45, YFG-65, YFG-75). As my YFG-15 has the exact same head size as the YFG-30, I am having some serious reservation that size was a major factor in Yamaha's numbering system.
The example shown above (serial number 789389) is actually one of two such frames I've come across in as many weeks; the other one is numbered 811837. Assuming that Yamaha racquets were consecutively numbered for a given model, that would mean that 10's of thousands of YFG-15 were produced. If so, surely there must be more than a handful of these still floating around? The basic specs as measured are quite close between the two long lost siblings that I have now reunited (only 4 grams apart in weight, 2 mm difference in balance point), so Yamaha QC did a reasonable job on these, which may have been better than that of the YFG-20 that Coachrick had experienced.