Last time I checked, the view of the cops (yup, they make shit up as they go along...) was that if it attaches to a firearm, you need a permit.
It is not hard applying for a permit, download the form, do it in digital form so you have a backup (in case it gets lost...) and submit.
You could of course also try phoning your local arms officer, or if you have lots of time the central firearms unit to get an answer.
If it was me, I'd just do the paperwork as a precaution.
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