VSL is Val St. Lambert / Belgium.
If Ivo (thank you!) can look it up it seems best just to be patient and wait until he´s near to his books.
Otherwise you can try to do some detective work yourself and find books on this topic or old catalogues.
Some VSL reprints e.g. are available on ebay.fr if I remember correctly or you need to find a good library
with interlibrary loan or visit Corning Museum....