The EZ's were around 2014 and "labelling" changed around about then. I had to do a quick search as I couldn't remember.
The lack of any number probably means they were a Std build set.
https://mizunoforum.com/threads/mizuno-jpx-pro-serial-number-question.25052/#post-122587
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I once enquired about this to Mizuno via email as my previous set (Custom fit) had serial numbers and my 2nd set which were standard spec from a retailer had no serial numbers on the heads, this is the reply i got.
"Good Afternoon,
Our standard sets are sent in plain boxes, and standard sets often don't have a code on the hosel or ferrule. (Custom fit will always have a code on the ferrule/hosel and will usually come in the branded box)
Codes on the shaft are from the shaft manufacturer and do not relate to Mizuno."
Regards
Custom Fit Team
Mizuno Corp UK
Mizuno House
612 Reading Road
Winnersh
Wokingham
RG41 5HE "