A key can be transcribed in either code -- UEB or Nemeth. In your sample, every item being keyed is Nemeth, so I would suggest that you open Nemeth after "below:" and terminate Nemeth right before the closing TN indicator.
In Nemeth, an alphabetic key consists of two or three lower-case English letters. [NC 26.9.3] Single-letter keys are not mentioned in the Nemeth code. You could use a numeric key (with upper-cell numerals).
[FORMER TEXT REMOVED. SEE FOLLOW-UP RESPONSE.]
Keys read the same in either code (upper-cell numbers for a numeric key; no contractions for an alphabetic key of two or three letters). You do not need to insert an opening Nemeth Code indicator before the graphic.
- Lindy