Hi Herman,
Thanks so much for your very informative reply and studying the photos you have posted I can certainly see the similarities of this maker and the unmarked knife I have, I was aware the late plastic straps aren't very common which is actually one of the things that attracted me to it. I have seen one on a UK dealers site with the same straps marked RZM M7/36 for Horster he states it is only the third example with the late plastic straps he has had in 30 years of dealing so I guess they are actually quite scarce. Lovely group of knives you have posted they all look in very nice condition and I specially like the one with the engraved pommel. I will take your advice and leave the press stud open from now on and I was also warned about this problem with these plastic straps by the dealer who sold it to me, I paid ?275 English pounds for it which I think was a very reasonable price though it's not in fantastic condition but the plastic straps are good and totally intact.
Many Thanks. Regards, Andy.