Sorry for the late reply -- You may have already found this info ...
There are 4 Thevenet gunsmiths listed in various French sources. Two in Saint-Etienne - one in 1780s and another 1840s - maybe Father/son. One in Paris with no dates or details. The other two Thevenets are later, like 1960s.
It appears your pistols are likely made by the St. Etienne maker listed around 1780. The listing shows him in the Loire valley, but no mention of Lyon. It does say he made pistols for the "marine" (Navy). Interesting that the side plates and lock plate appear English style. Where is the signature located?
Not much, but might help.
Patrick Hornberger
eastwind@wildblue.net