I'm not sure it was legal to sell them to me as they're clearly marked "Flensburger" and therefore must be deposit-bottles from the brewery.
ETA: What are such bottles called in English? -around here they're called something like Patent-cork bottles
There's a few breweries that have used this bottle design. Grolsch is one of them that comes to mind that still does. I've seen it called "flip top", and "EZ cap", but it looks to me like "swing top" is more commonly used.
I highly doubt you have anything to worry about legally. Micro breweries that use a bottle deposit system have the bottle cost in the price, which is credit back towards you next purchase when you return it.