Ken, 3.5 mm is perhaps not too thin for a HEIANJO TSUBA I think, but it would be too thick for an early KATCHUSHI or TOSHO TSUBA. On the other hand, there was not much sense in repurposing an old TSUBA plate, as making a new one was not so much work. In my understanding, using an older plate for decoration was not very often done, and in many cases, you wouldn't end up with a much better or more beautiful TSUBA. Just my two ¥.