We extend to you a cordial "WELCOME ABOARD !" Come on in, make yourself at home, we are a friendly group of enthusiasts, and we also appreciate the classic Chris Craft Roamer, Corsair, and Lancer boats too , as they are all on the same family tree and share much in common !
I don't know the answer to the questions you asked, but I do know that if you built a tuning fork out of lead it wouldn't have much of a tone because lead is dead so to speak. Lead has basically no vibration at all, so on one hand someone might think it was good at dampening vibration, but on the other hand it seems like that could have been the result of a shop or a previous owner trying to deal with the issue. Having that much lead just seems nuts. If the boat also has the steel harmonic vibration dampener too, then this could have been an owner installed issue. I don't think it would have dampened any real harmonics as much as it isolated the noise; big difference between dampening and isolating. That lead you found just makes no sense at all.
However, now that you have also verified you have two 23 Lancer inboards with the steel plate, I think we can say for sure at least the 1973 and 1974 versions (mine is also a 1973, I think Mike's is a 1973, and Jerry has a 1974 too) got the big steel tuning fork.
Chris Craft probably did a sound reading or other technical measurement to determine the frequency, then had an engineer look at how much unsupported steel in section modulas hanging out there with no supports on the ends it would take to start vibrating on its own and counteract the frequency being generated by the boat. Again, craziest thing I've heard of in boating for quite some time. Hey, maybe my boat is now singing at a little different note now that my steel plate is a bit corroded? Maybe a B-flat?