After some delays due to asynchronous holidays etc, I received the backsight slider for a very modest price from bigduke6 and have now installed it. The MkV now looks a lot more convincing:
- but for anyone who also has the problem of a MkV without a backsight, here's how you can attach a No.4 backsight, without making irreversible alterations to the Mk.V.
Just a temporary fudge of course, but in my case "temporary" was 8 years! Note the spring between the left lug and the No.4 backsight, to hold the sight over to the right.
3 (at least) cheers for these forums, and many, many thanks to Geoff for his help!