I believe they are (generally) genuine. Whether they are "legit" is another question, which applies to a huge number of items on websites.
Yes, you can fit them yourself within Openreach rules. Their design is such that you carefully remove the NTE5A, (that's the faceplate), plug the new bit into the test socket, and positioning any wires attached to the faceplate into the slot in the new filter plug the faceplate into the socket of the filter.
It comes with longer screws to replace the originals. Screw them in, not too tight, job done
. This page
shows how it all fits together, though starts from it already being there.
Bear in mind it disables broadband at all extension sockets. If you want to use an extension, which in broad terms is not recommended for FTTC, this can be achieved by doing a bit more work, that is also normally within the rules.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
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