In an earlier contribution to this discussion,
GMM wrote:


Thanks chaps, but that's what I've always been sort of used to, and
these ones do have a couple of opposing notches, but also have a key
slot. Being a flimsy plastic thing, I'm a bit reluctant to heave on
it too hard, but it seemed pretty solid when I tried a couple of big
screwdrivers, so I reckoned that the key slot thingy (like a flat oval
hole with a circular middle. about 1.5 inches end to end) must be
doing something.

Care to upload a photo somewhere, and post a link to it here - in case
anyone is able to identify the type of cover you have?
--
Cheers,
Roger
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