Hello Argo,
Waiting for the pics, but:
"levier" or "cl" is normally the opening lever.
Pontet is fixed (trigger guard) and normally cannot be moved unless it is also the lever (e.g. like in some Purdey rifles or Win rifles).
"Pontet volute" does exist, but it is fixed, and not a mean of opening.
There should not be a "pontet serpent" but a "cl/levier serpent".
"Serpent" is of course "snake" and a perfect example would be a Grant side lever.

Ads are often whimsical and/or misinformed in descriptions, so should not be used as a 100% valid source of info (nobody is perfect).
Even catalogs are rife with misuse of words (but certainly better than private ads).
Best are old reference books from the 19th early 20th century, such as the one from Lebeau-Courally or Nouvelle etc.
Best regards,
Martin