When searching for information about Gustave Boël I found this article on wikipedia.com. At first this seemed to be a perfect match: From Belgium, metal and coal industry. But it is not possible. Since he died in 1912 it must have been another Gustave Boël who send the Katarstky covers in 1920. Perhaps someone from the same family.
Here are some recent pictures from the abandoned blast furnaces of the ‘Usines Gustave Boël‘ in La Louvière in Belgium.
Perhaps some of the readers can shed some light on this puzzling matter.