I've got a similar set of cheapo ND filters (in Cokin P mount) quality seems reasonable for those, but they don't have any need for a screw thread.
ND8 should be 3 stops BTW, 1/8 as much light, my set work out roughly right according to the spectrometer at work.
ND=2^(stops)
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Mike
Pentax K5ii & Panasonic G5 user (with far too many bits to list)
Member of North Essex Photographic Workshop
Also online with PentaxUser.co.uk, Flickr, MU-43, MFLenses...
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