Other responses have already indicated that the "5+" is a (less-than-ideal) attempt to transcribe the slashed-5 of the original publication. The question then becomes: what does the slashed-5 mean in the original?
When reading period sources, familiarity with instruction manuals from the same period and country is often useful. In this case, we can find the answer in Michel de Saint-Lambert's Nouveau traité de l'accompagnement du Clavecin, de l'Orgue et des autres instruments published in 1707. This is available on IMSLP: https://imslp.org/wiki/Nouveau_trait%C3%A9_de_l%27accompagnement_(Saint-Lambert%2C_Michel_de)
On page 11 of his manual, Saint-Lambert writes:
Le 5♭ ou ♭5 marque la fausse-Quinte.
Le [slashed-5] coupé marque aussi la fausse-Quinte.
The 5♭ or ♭5 marks the "fausse-Quinte" ["false fifth"].
The [slashed-5] cut also marks the "fausse-Quinte".
So according to Saint-Lambert's practice, slashed-5 means the same as flat-5 - what we would now call a diminished 5th.
Continuing, on page 14 he writes:
La fausse-Quinte s'accompagne de la Tierce, & de la Sixiéme.
The diminished fifth is accompanied by the third and the sixth.
Saint-Lambert then gives a caveat to this rule:
La fausse-Quinte s'accompagne de la Tierce & de l'Octave, comme la Quinte juste, lorsque la note suivante au lieu de monter d'un semi-ton selon la coûtume, fait un intervalle plus grand, soit en descendant, ou en montant.
The diminished fifth is accompanied by the third and the octave, like the perfect fifth, when the following note instead of rising by a semitone as is the custom, moves by a larger interval, either descending or ascending.
The case given here fits Saint-Lambert's rule perfectly, we have a slashed-5 figure in which the bass note rises a semitone, so we add the diminished fifth, a third and a sixth (C natural, A, D). In modern theory we have a very conventional first-inversion dominant seventh chord, with the bass note functioning as a leading tone moving to the tonic ("rising by a semitone as is the custom").
Another source for this rule in French music is Jean-François Dandrieu's Principes de l'Acompagnement du Clavecin (1718): https://imslp.org/wiki/Principes_de_l%27accompaignement_du_clavecin_(Dandrieu,_Jean-Fran%C3%A7ois)
Under his Table pour s'exercer sur l'Acord de la Fausse Quinte ("Table to practice the chord of the diminished fifth"), Dandrieu writes:
Cet acord est composé de la Fausse Quinte, de la Sixte et de la Tierce. On le fait comunément sur la sètième note du Ton, que l'on nom Soufinale, quand cete note remonte à la Finale. On marque l'acord de la Fausse Quinte de l'une de ces deux manieres [slashed-5]. ♭5.
This chord is composed of the diminished fifth, the sixth and the third. It is commonly found on the seventh note of the scale, which we call the "Soufinale", when this note rises to the "Finale" [Tonic]. The chord of the diminished fifth is marked in one of these two manners [slashed-5]. ♭5.
From my own practical (but most definitely amateur!) experience of playing continuo in French music, Saint-Lambert's caveat about larger intervals rarely occurs in practice, and my first instinct is to play a 6/5 chord whenever I see a slashed-5 symbol. We can see this in the Hotteterre. The next bar has another slashed-5 on an F-sharp, here the bass noodles around a bit before finally arriving at the G, so another 6/5 chord would be appropriate. In the following bar there is a slashed-5 on a C-sharp. Here the bass falls to an A before rising to a D, so strictly following Saint-Lambert we wouldn't add the sixth, but the C-sharp is clearly functioning as a leading tone to the D, so a 6/5 would work again. [Edit: But I note from Saint-Lambert's bottom-left example in the image above, which also has this pattern, that he doesn't add the sixth initially, but he makes the chord over the C into a dominant seventh. I'll try to remember this when I'm playing in the future - maybe it will sound more "French".]