Straight out of the box with uncut steerer and including X-Loc: 1536g:
Both of my forks are QR and my SID Race has been upgraded with the dual flow rebound damper. Basically, these forks feel pretty much identical in terms of stiffness, tracking and small bump compliance.
The 120mm SID XX feels great ran with 30% sag and 70psi air in the positive and negative chambers. I run the 100m SID with 85 psi in the pos and 65psi in the neg getting 25% sag.
I dropped my bars by 10mm to compensate for the slightly higher front end.
The big difference between the two forks is the extra 20mm travel on the 120mm fork makes things feel just that little bit plusher on bigger hits.
The X-Loc is a closed hydraulic system using 2.5W fluid. Pushing the button in on the X-Loc operates the master cylinder which then moves a slave cylinder connected to plunger rod in the XX damper. The plunger rod in turn, pushes open an orifice at the bottom of the compression damper (the blue poppet valve in the pic below):
When this valve is closed, it provides the pedalling "platform". When the valve is open, like in the picture above there is no platform.
When closed and providing a pedalling platform, the valve will still open if the bump force is great enough to compress the damper body itself.
Adjusting the gold coloured +/- gate knob on the X-loc remote adjusts where the adjuster rod ends up. The more extended into the damper, the less bump force is required to open the poppet valve and get the fork moving.
Here's what it looks like with the X-loc remote button out - you can see the adjuster rod is back from the poppet valve end. The damper cartridge would have to compress by the amount of this gap before the compression damping circuit would open up:
In comparison, here's with the button pushed in - the adjuster rod is making contact with the back of the compression poppet valve, pushing it away from the bottom port of the damper:
This set of forks came with the 20mm all-travel spacer installed. To remove it and get 120mm of travel out of these forks is the same process as for the 100mm SIDs outlined here.
Here's what the air piston rod looks like with the all-travel spacer still in place:
The fork comes with a 20mm and two 10mm all-travel spacers. The fork stanchions have sag markings for 120mm, 100mm and 80mm configurations, so its fair to say that travel options from 80 to 120mm are supported. The 10mm all-travel spacers mean that you could configure this fork for 120mm, 110mm, 100mm, 90mm and 80mm. With additional spacers even less travel would be possible.
Rockshox service manual specifies 106ml for the damper side.