It seems to be the reciting tone in both cases, as @ClemensRomanus said.
The custos is a singer's aide. I can imagine it being helpful for someone to play the reciting note (as opposed to the starting pitch) before beginning singing.
Note also that it isn't the usual custos with an ascending stem on the right side; instead, it is an inverted custos-like figure with the stem descending on the left side. I'm not sure it should even be called a custos.
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