Well from what I am seeing this may be a dead end but what the heck... I have a few details that no one has mentioned that may make the difference... may not.
I have an SZL Triangle original etching of Cartier, Two Arches... created in pencil and water color on brown paper and signed in pencil. I it is a beautiful piece that I rescued from a estate sale for $1 over 10 years ago and had it preserved. I am curous too what its value could be and/or its histoty.

There's a whole bunch of threads concerning SZL and the Paris Etching Society (https://www.askmehelpdesk.com/search.php?searchid=951448).
Unfortunately they aren't worth a great deal, a few hundred at most.