Found some info on a past post here on MC, definitely between WW1 & WW2, trying for more info but this may take a while, definitely after the supposed early 1900's end of production date.
LINK :: https://marbleconnection.com/topic/17876-some-nice-and-colourful-single-pontil-marbles/
Hi Hansel
That is a really interesting leaflet. I tried to do some research in the internet. I found this book here in which all the glass-manufacturers in Germany were published, the "Glasfabrik Wernerhuette" too. The book was published in 1907.
http://archive.org/stream/deutschlandsgla00unkngoog#page/n2/mode/2up
But more interesting is the fact, that on "your" leaflet is mentioned that they also produce glass-parts for radios ! According to google the first radio-station in Germany was on air in 1923 ! So the leaflet has to be published later, my guess is between 1925 and 1933, when Hitler was elected for chancellor.
So this leaflet proves that marbles like sulphides, onionskins and swirls were definitely produced between WW I and WW II which is very interesting because I (and probably most other collectors) always have thought that no such marbles have been produced after WW I !! So the time range for these marbles has to be approximately from 1870 to 1930 or so.
Really really interesting, thanks a lot for posting it !
Ron