Je vous salue mes petits Gaulois. Some things never change, do they?
In order to tell if a mei is genuine without doubt, it is necessary to have reliable reference material. In the case of ShinTo or ShinShinTo blades there are usually enough swords surviving from every single smith to tell the difference. Going further back in time things become more and more tricky and speculative. If we're lucky, there are many signed specimen left as by Osafune NAGAMITSU or Rai KUNITOSHI f.e.. It is different however in the case of Hosho SADAMUNE. In Hosho-school there were several Toko at the end of Kamakura period working on a very high quality level, but with hardly any INDIVIDUAL traits. Among them SADAOKI and SADAYOSHI are quite well documented by several signed blades each. Unfortunately there are hardly, if any at all, signed specimen by SADAMUNE surviving. Every newly discovered blade with (Hosho-)SADAMUNE mei is put to quality test first. Here's where most of them can be excluded as gimei, for it is extremely difficult to copy top-class Hosho swords. Even shodai KUNIKANE from Sendai clearly failed. Now, when a new blade with SADAMUNE signature, made in the style of and with the qualities of Hosho-school is showing up, it cannot be attributed to SADAMUNE easily because of lack of reference material. One single mei for reference, even if it is a JuBi, is simply not enough to make judgement waterproof. As I tried to explain, this is because of missing individual traits within the Hosho-school: The blade could have been made by SADAOKI or SADAYOSHI and turned into (more prestigious) SADAMUNE later.
As far as Juyo Bijutsuhin is concerned: Understanding of NihonTo has improved during the past sixty years. Pre-war standards like JuBi cannot be considered the final and undeniable truth in every matter. This goes for some (not all) of old(er) literature as well.
reinhard