Would these be the Mystery Slings?
Two minor varieties with identical brass ends, one with unribbed web edge, one with a slight rib.
One sling seems to be marked "ABL" over "??" over "1954" and another "GIE" over "AL" over "1954"---these are the two with the slight ribbing. The thinnest type I have never found marked. I think the end tabs tie these two types to the same source, by the way.

So, Belgian?





-----krinko