I would make a distinction between a proof and a specimen of a banknote.
A proof is a pre-production printing which can be part or all of the final design of a note.
A specimen is an example of a production design with a face value of zero.
I would definitely argue for leaving proofs of banknotes in the years section. For some countries there can be a lot of variations of proof notes, far more than of the issued note (some issues of Ireland are an example of this). The listing would be cluttered up with proof notes.
I would also argue for leaving specimen notes in the years section, in that they are an example of the design of the note, and as such there is no variation in them from the issued note design. Splitting out specimen notes as separate entries would greatly increase the number of listings for some countries (Finland would grow by around 50%).
There are also non-production specimen notes to consider, those of TDLR for example, which were produced of issued note designs in non-standard colours on unwatermarked paper for use in their Specimen Books as examples of the company's work. These exist for many countires.
Again, I would leave these for the year lines.
Also, in the case of rare issued notes where an example of the note is only available in specimen or proof form, it is useful to be able to use an image of the specimen or proof in the picture slot of the entry.
