Hi Fabrizio:
You are probably correct. I was concerned a little bit that the tail light portion was not quite "round" enough to be 1959/1960 Buick style but it did not look "horizontal" as the tail light on an Impala is. As you have the original photo, it may show more detail.
BTW, one of the cars of my youth was a 1959 Buick LeSabre Convertible.
I made sort of a "Street Rod" out of it with twin 4 throat carburettors, racing exhaust and some other modifications. Also some very fancy road wheels. It used to really surprise some of the Ford and Chevy guys with it's speed but it was a pig on twisty roads.
I am still looking for the answer to your trailer puzzle, who knows...............I might find it yet.
Bill