Quote Originally Posted by jonh172 View Post
Hi guys!

Does anyone know what this slot is that is milled in this No4?

Thanks!

Jon

https://imgur.com/a/gG4t2Jl
It's possible that it was some form of deactivation, or because the receiver is thin there, it cracked thru and someone milled the crack out to stop further progress of the cracking.

That is what I would suspect.