The cut off in the Mk V doesnt have a viewing hole. I have a cast nose cap band from Blackley and sons in Englandicon, took me quite a few months of wait time. It's in the raw, but if you're interested in buying let me know. They take a bit of work.

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Its hard to tell in the pictures but it looks like you might be able to splice the front of a Mk III forend onto your forend, that would be better than using a complete MkIII because of the screw hole and inlets for the back sight protector. The rear handguard is the other tricky part, but you can cobble together a Mk Iii and a No 4. You might find it easier to find EFD a replacement wood guy.