I have about the same age Renault&Chatelain single-action harp. Very tender instrument.
Did you get this harp after restoration or before? If before, you sould be extremly careful.
I can advise you with strings by puting in touch with Beat Wolf and Bernd Kuerschner. Beat Wolf is the world top specialist in single-action harps (he even makes them), Bernd Kuerschner makes the best strings for harps ever. Especially for historical harps. As far as I know, most of historical harp makers and early harp players in the world work with Kuerschner. I recommend you to write him for advise and strings order.
It’s absolutely right: the string tension was a lot softer in that times. Unfortunatelly, I have a bad example of wonderful Naderman harp, owned by one collectioner, seriously damaged by puting too heavy strings on it, couple of such damages I restored and lots more I heard about. The soundboard just went off. And that is good that restorer didn’t change the glue: old glue just fell apart and luckily the instrument didn’t crush completely.