On the No4 Mk1, there is what appears to be a square piece of metal sweated into a recess in the receiver, see the attached photo.

I've been unable to find anything on the forum which explains this. I'm pretty confident that there is something on the forum that answers my question, but I can't find it. I meant to ask this question some time ago but decided to search the forum for an answer first. I look forward to the responses
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