The excerpt is taken from page 6 (IMSLP publication). Does anyone have a clue what purpose those circumflexes have?
It meant "pluck this note with the thumb" - but I don't think this notation is still in common use.
Mertz published a Guitar tutor (here, in German: https://imslp.org/wiki/Special:ImagefromIndex/442101/torat)
The scan of page 10 is pretty low quality, but from the text of the "1ste Regel" (Rule No. 1) and the exercises that follow, even though the "^" symbol seems to be missing from the text of the "rule" itself, it seems clear enough that ".", "..", and "..." denote the first three fingers of the right hand, and "Daumen" is German for "thumb".