As I want to experiment with tonal options, I'll be receiving a loaded SSS pickguard (with Bare Knuckles and normal configuration) and therefore replacing the SCN and S1 thing.
The thing is I'm not an experienced solderer, nor do I know exactly which wire goes where and why. Of course I' ll have a pro do it if need be, but just out of curiosity I'd like to know anything you might think is important, eg grounding, what exactly will I have to use solder on (and why please), any ideas, thoughts recommendations welcome, thanx a lot.

Take it to a pro- Martian is the wiring guru, im sure he will see this thread and comment

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Preloaded assemblies are already put together!!!!

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

The only thing you should need to do is connect the ground wire to the trem claw.

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I'm not an expert, but I play one on the internet.

OK. in the original assembly there is a ground wire (ringtip ending) screwed on the bridge pickup cavity, soldered on the pot in the middle. I guess this has to do with shielding paint being effective? What do I do with it? I understand that I will have to solder the hot and ground on the jack socket and the trem claw ground, but what about this 4th connection? Is it necessary?

You will have to connect a pre-loaded pickguard to your input jack. There are two wires for that.

The white wire (or red somtimes - hot) solders to the tip connection of the input jack. Follow the tip of the input jack to see where it comes out at the base of the input jack and follow it to the solder connection to learn which connection is correct. The other end of the white wire connects to the middle rung on the volume pot. Allow for enough length of wire to comfortably reinstall the pickguard.

Black is ground and solders to your common ground grouping which normally is found soldered to the top of your volume pot. The black wire solders to the ring connection on the Input jack. Sometimes in a preloaded pickguard, both the black and white input jack wires are already soldered to the pots.

You will also have the ground wire from the trem claw which you can solder to the common ground grouping on the volume pot. You most likely would remove the trem ground wire from your old wiring assembly by desoldering it from the common ground on that volume pot. You can also snip it at the end if you are not worried about the old pot. Strip a small end of that snipped wire and resolder it to the common ground group on the volume pot. The ground wire that connects to the body will also ground to the common ground on the top of the volume pot.

Depending on the type of wiring for the loaded pickguard, often each pot will have a ground wire soldered to the top of each and connecting to each other and running to the common ground spot.

To answer your question about the body ground. If it's there, use it. Just make sure that your solder connections are solid. If you are not familiar with soldering you could end up doing more damage if you leave your hot soldering iron on parts too long. Hope that helps. It sounds confusing but it's really straight forward.

When in doubt, look at your old one before you remove it and either shoot a close up photo or write down where all the wires lead to that you must remove to get the old assembly off

Very informative, thanks a lot. I installed the new pickguard without using the shielding paint grounding and everything works smoothly. I didn't really feel comfortable desoldering it from the original pickguard.