On the question of the f stamp, this shows the federal German police that it isn't a fire arm. Basically anything capable of firing a round/projectile that hasn't got a stamp is treated just like an unlicensed firearm and the police are entitled to use the full force of the law (marker confiscated and a big fine I think). Ferries and the euro tunnel are usually cool with taking marker across and no need to remove the reg as long as the tank is depressurized. Taken my kit back and forth numerous times as I've just moved back from Germany. Does help to have PGI or the like on top so Customs know what it is if you are pulled
Hope this helps.