Term for a perfect specimen or sample

I'm trying to remember a word that is used to describe a perfect specimen or sample of something. That is, a unit that best represents other units.

I think this term may come from stamp collecting. My mind associates this word with words like supernumerary, nomenclature, and nominal. It may even sound like one of those words.

The intended usage is something along these lines, "An uncompressed XXXXXXXXX file was then transcoded to numerous candidate codecs and objective measurements were made between the XXXXXXXXX and derivative files." In this example, XXXXXXXXX would be replaced by the word I'm trying to think of.

Many thanks in advance for any assistance you can offer!

The word you're looking for is exemplar. Here's a philatelic usage about the famous 1918 "Inverted Jenny" US airmail stamp:

An exemplar of it was bought for $ 977500 in 2007.

How about archetype?

M-W:

archetype: a perfect example of something

Your example:

"An uncompressed archetype file was then transcoded to numerous candidate codecs and objective measurements were made between the archetype and derivative files."

Although deadrat's exemplar is the word that you were looking for, I think that in the context you describe, perhaps reference is closer to the mark.