I went through a phase of buying these up a couple of years ago of e-bay usually between £15 and £20.
At the time I was fishing for catfish with an open bail arm and opo's are one of the only alarms that work effectively with this method.
I forgot how unreliable they can be. Especially in the rain!
I still have a conversion done by a mate in 1990 with a G.P.O speaker that works reliably even in the rain.
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