Totally possible that the silver tag changed...
Yes, there are two silver tags. The one you're thinking of was in use from 2009-2013/14. (Before 1987 there were no tags; the black tag was introduced sometime in 1987 and was in use until 2008 (with the starting letters G, H and I from 2006 -2008).
It's possible that the Mizutani SZLC was the first
new blade that featured the new silver tag. The old one had wings of different sizes (the left one being smaller than the right one), and both wings were smaller than the current ones, allowing for a wider angle between them. On a side note: The green and red dots (green for composite and red for allwood blades on the bh side were introduced in 2016. So, the SZLC form 2014 and 2015 had the starting letters O and P and featured the "Ferrari-like" lens on the bh side of the handle until it was replaced in 2016 with the green dot.
Hypothetically speaking, a Mizutani SZLC with the starting letter N, a new silver tag and the green dot on the bh side would be a de facto fake.
Since OP mentioned that he paid "a fair few £££££’s" for it, I wonder what he will do with the information given to him. After all, he knows the person who sold the blade to him. If that person would be my friend/club mate he would get a serious talking to from me.