Sig now has a drop in universal manual safety kit for the P320 that doesn't need a special cut to the FCU.
For those that want to add a manual safety to your old non safety P320s, but don't want to cut the FCU. Now there's a new manual safety kit from Sig that is pretty much drop in and doesn't require a cut.
it does require the manual safety style trigger bar though, but you can easily add one of those too if you don't have one. That's what i had to add.
It works by using a special plate to hold the detent of the safety instead of a cut in the FCU. I tried to capture the plate in the pictures.
All the tools I used to install are in the picture. Basically a flat head and an old lollipop. The lollipop stem was used as a slave pin.
I used Sig Mechanics youtube video on how to install a manual safety to give me the gist of it. Just gotta add the extra step of adding the plate. Hopefully you can see where the plate goes in the pictures.
Unfortunately its out of stock already but keep an eye out for restock if interested. Link to the safety: