He couldn't remember the names of 10 stops.
10 stops? Does that seem right?
10 stops is not the limit, though. In western part of Lithuania we have a chapel of the Franciscan church in Kretinga, where stands an organ from ca. 1680 with 8 stops and yes, the organist there applied stickers with the numbers. If I remember correctly, this time they are bright green. You saw the date right - this is a state-protected historical organ...
What's the smallest organ on which stops you think writing numbers would be helpful but not debilitating?
Transposing Sequence in Bb Major: IV-vii6-I