With grateful thanks to DEBEN for this, which clears up an issue for many of us:
The threads are different between older and newer models:
"There is a bit of a divide between our scopes, where we've made efforts to simplify sunshade and accessory fittings. The older models (Varmint SF, Panorama EV and Nite-Eye Digi) will all use that thread pitch you've got working. The newer models (those that can be seen on our website) will use the M38 x 0.6 thread.
With regards to the Sidewinder, there's two different eras of the scope which have different thread pitches. The newer models are supplied with metal flip-up covers and will have the M38 x 0.6 thread. The older models were supplied with metal screw-in caps and use a different M35.8 x 0.6 thread"
And further info:
Another easy way of telling is to inspect the scope's logo. If the side of the eyebell has the newer logo on it (attached) and pictured here, then (it will be) the new style scope with the M38 x 0.6 thread.
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I had contemplated getting some screw in adaptors made for the sidewinder because its popular with NV users, but given the above information, I'd need to make two thread versions and feel it is not going to be a viable proposition.
I am bearing in mind that the eye bells are uncluttered with IR modules and that both my slip on adaptors and pending replacement collet chuck adaptors will be suitable for both old and new spec Hawke scopes.