Hy all
First and for all: I got the hint about the pictures. Thx for letting me know.
Regarding the knife: the handle is exactly 9.5 cm long and 1 cm wide. The complete knife opened is 17 cm long. The bolsters are indeed sheath material that is worn on some corners. Underneath seem to be some kind of resin / the motive is only in the plate material. Closer inspection shows me there used to be a 2nd blade (on the opposite side) / possibly silver. Unfortunately it broke off near the ricasso. No maker markings on the knife / blade.
As for the location of the market: found it in Brussels / Belgium / Marollen flea market (it's a daily market on a square called 'Vossenplein', part of the Marollen area). The best of all: I did not even bargain on the 3 € asking price
I received some more info on the knife: is is supposed to be French / made in Paris / 18 th century. Have a look at the hopefully correctly inserted picture (book from Dominique Pascal - Coutellerie de poche)