Your shotgun was made by Imman Meffert, which was a top of the line operation. The Krupp steel barrels being prominently advertised with a rollmark would indicate manufacture in the 1920's. Krupp Werke of Essen, Germany manufactured the steel for almost all the 1920's-1930's shotguns and drillings made in Germany, and the strength of Krupp steel was somewhat legendary (to say nothing of them making artillery pieces for over a century...hence the crossed cannons). "Sicher" means "safe" or "secure" in German, meaning your gun was made for the German market, not marketed for a foreign country (I think...sometimes they'd stick "sicher" on a gun meant for export).
What you really need to do is open up the gun, remove the barrels, and take a picture of the barrel flat right beneath the chambers. This will almost certainly have the last two digits of the year of manufacture.
I'd also get your chamber measured. While your shotgun is a strong, sturdy boxlock design that can certainly withstand a lot of pressure, it is by no means invincible. Your shotgun, if I had to guess, has 2 9/16" chambers. That means you can't shoot modern ammo through it. But fear not! Go to www.polywad.com. Very cheap shipping rates and they make 2 1/2" and 2" shotshells, which would both fit your Imman Meffert just fine.
I'm green with envy, incidentally.