I sow this cartridge posted at
http://members.shaw.ca/cstein0/revolver.htm
What gun used it?

Cristi, the only 6.5 rimmed round I find in my references is a 6.5 Mannlicher for the M1894 Auto Pistol. The case length, however, is slightly longer than 23mm. I'd suggest contacting the author of the site as he would be the one most likely to have the data: cstein0@shaw.ca

DWM lists a 6.5 revolver cartridge case no.507 which may be the 6.5x20 Romanian.

If anyone has a good reference book on Belgian made pistols that might be a good place to look.
In the late 1800's and early 1900's Belgium was very prolific in the manufacture of oddball pistols.

WC