Regarding the material, the Ikeda Labs website is particularly uninformative, only saying "Hybridization of the material of the main part" (Google translation), which seems to indicate that it's not pure stainless steel or aluminum.
http://www.islabs.co.jp/it345cr1.html
Now the ''curious'' anti-skate provision by 345&407. This is
not even similar with FX kind. The forgotten ''child'' the 245
however has similar to FX anti-skate provision.
I have an FR64fx and an FR64s, but have not compared directly with 345 & 407. However, from every picture I can find, the anti-skate in 345 & 407 looks very similar to the FR64fx. Here are pictures of 245 and the anti-skate looks quite different:
http://www.hifido.co.jp/sold/?KW=&G=0302&LNG=E&O=1500&L=50&C=07-30498-90074-00
I can't think of what aging or maturing would do to silver and why it would sound better. There has been improvement in casting, at least for copper, since the introduction of FR64s, so it might well be that current silver is better made.