A known tube seller with a well supported supply to the UK took on a usually seen approach from a sales person when questioning the era of a NOS Tube from a later production era, vs a earlier production era.
I was pretty much told I was being mis-informed and with them being an authority, they should know that their Later NOS Production Tube is on Parity or even better that the earlier Production.
That story probably lasts until earlier production models are discovered and offered for sale at a much increased price, and a inquiry is made about such a Valve.
I don't think the parity word will be used in a description, unless it is used to compare to another Brands Tube from a similar era, and commanding s similar price.
I found the Tubes over a period of Time as a early 60's Production, and when Tube Rolling compared them to 70's Production, which the latter were quite inferior in the presentation and no where near as attractive when in use.
Treading Carefully and being very selective about where one take their influences will help steer toward a successful NOS experience.
My time now is spent looking into the reports of New Production that fairs very favourably to NOS Tubes. There are some very positive findings to date.
These latest versions of tubes being discovered, will for myself, most likely be the purchases made for the future Tube exchanges, as finding very good, close to Manufacturer Spec', Matched in Measurement Tubes, from a 40-50 year old era is almost like going on a search for finding Rocking Horse Poo.