I own (4) Belgian posters at the moment... Once Upon a Time in the West (which was my first poster!), an original 1936 Modern Times (Chaplin) and I recently got Barbarella and Two Mules for Sister Sara. They are fun posters to collect, however, the consistency seems weird. OUATITW is on super glossy stock, while Barbarella is almost matte.... they are both from the same year!
I suppose your OUATITW is a early 70s re-release. Does it state CINEMA INTERNATIONAL CORPORATION in the credits ?
There must be a PARAMOUNT release poster, although I never saw it.
I love the belgian posters as well. I have only one frame for them (I switch posters once a year), but I have quite a few, mostly
on the talents I heavily collect like McQueen, Wayne, Western All'Italiana and many more.
Here are all 14 PECKINPAH posters from Belgium. Some I took with me while filming PASSION & POETRY, they are easier to transport (for signing) than 6-sheets