A case for Gibberish

[-JKP-]

 doesn't matter, it could be a reference label, an abbreviation, or an adjective rather than a noun

Yes, I would agree with you that JKP's explanation is not a sufficiently persuasive one. If it is a reference label, how and where can one find the thing to which it refers? If it is an abbreviation then it would have to be so brief as to be virtually useless in identifying the word it is abbreviated from, which is particularly significant in the case of the often very specific drawings that labels are attached to. It is hard to see how the same adjectives could be applied to such specific drawings. Another point worth noting is that distinct repeated labels often have very similar spellings.
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It's not an explanation. It's a list of some of the possibilities. Labels might be something other than nouns. In other words, the seeming incongruity of the same label being on seemingly different images is only an incongruity if you make assumptions about what the label represents (or make assumptions that a possible cipher system is the same for each iteration).

• Where can one find the thing to which it refers? It might be a location on a page. In the middle ages, they did use location markers (section a, b, c, or d referring to a quadrant of the same page or of another page). They also used several different (and cryptic-looking) systems to refer to Biblical passages. Everyone knew what they meant. In Greek manuscripts numeric labels were quite common.
  
• Brief abbreviations are only useless if you are not familiar with them. I see brief abbreviations in thousands of manuscripts. They are very common. Maps often have them also. Words are very frequently truncated or chopped into pieces.
  
• It's very easy for different adjectives to apply to different drawings. Words like hot and cold, wet and dry were applied to everything in the middle ages. Verbs, also. Words like grind, powder, mix, and soak.
  

I'm not making the argument that the labels are words (that's a different discussion). But we cannot make the assumption that dissimilar items can't have the same labels. They certainly can.