Is this an infill and why?

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

That's how they did the charger bridge. It's legit...and you can also see them after repair with a spot of weld to stabilize them there after rebuild.

The "insert" is a separate metal piece that was machined to make the slots and form for the stripper clip guide. It is cheaper to make it this way and then welding/sweating it into the receiver.

Thanks guys, very much appreciated. I really must have a closer look at my No4.

Regards,
Ian.