It's on my Cervelo RS bike. There's a hex nut at the bottom. I tried to reach down with a 4mm park Allan key, and try to untighten it to loose the wedge. It's pretty tight. I was turning counter clockwise to loosen. I might've stripped it. It's soft aluminum. Any advice? Thanks.

Not sure what S and H mean on the plug.

Assuming that's the top of your steerer, the bung in my felt road steerer(carbon) is a 5mm capscrew(so 4mm stick required) clockwise to loosen it it has a torque setting stencilled on it which i cant remember but think it was around 5nm so not excessive

I'd agree, clockwise to loosen. You are turning a nut on the bottom so that's why it appears to be reverse.
"S" could be Italian, stringere from Google translate but no idea for the H.

S=serrare=tighten. Also works en francais.....

https://customerservice.cervelo.com/

Too bad, I was turning the wrong direction! Why did they use Italian?! What to do now?

Got it out! Yes. The top hex edge of the bottom bolt is a little rough. I was turning in the wrong direction. I cleanen up the edge. Now the Allan key can go all the way through for better grip. I carved T and L on the top cap so I won't be mistaken again!

Ah , It seems just like the way Bar End shifters fit onto handlebars . the Head of the Countersink bolt is furthest from You.

[when reaching across the Hub to turn a spoke Nip on the far side of the rim (like using the brake pads as the truing guide)
you also reverse your thinking, though the thread is still RH]



'/,

Have you ever seen French-made pedals? The pedal threads are always marked "D" and "G". Heck, when I used to work in Quebec, they marked the water faucets "F" and "C".......

But, yeah, using the correct tool helps, too......