Hey GuntherHess,
The stoppers are not easily removed. I would be very careful about trying to force them out. For the most part, I have left the stoppers in the bottles when they have the rubber. If you really want to get one out, the safest thing to do is to remove the gasket. I have heard from a person who tumbles them that there are surgical type tools that will allow you to cut it away through that narrow opening.
I don't tumble personally. I was told that by a guy who tumbles. I suppose you can leave them in? It figures. I do think that the guy really cut them out that way, though.
The better quality stoppers were coated with 'block tin.' A few later companies used lead.
The one with 'LIQUID' on it was manufactured by the Liquid Carbonic Acid Manufacturing Co of Chicago that sol CO2 manufacturing equipment and later CO2 in cylinders and bulk along with a lot of bottlers supplies.
I have seen Hutches and crown sodas embossed with LIQUID or THE LIQUID. I had a couple of their supply catalogs around 1915-20 and they pictured their bottle styles including shoulder script Coca-Cola, Gay-Ola, and Bludwine.
I started with a photograph of the faded original. I scanned it and cleaned it up in the computer, brightening the colors to what the original must have looked like new. I sell 11" x 14" reprints at bottle shows and occasionally on ebay. I have a number of different soda ads I have done this with.