Forstner Bands reissue the Bonklip...

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You should have this open slot on the bottom of both endlinks.

Sometimes the slit is really tight- you just pry it apart gently to open the gap- which is why you see so many that have been mangled…”gently” is not in everyone’s vocabulary
There is a "narrow slot" that might be 0.3mm.

That there is a spring-bar slot implies to me that my Bonkip might be newer than some and after fitting to military watches, and I do not discount that there may have been two types at the same time.