Don't know what it is but I do know how it feels.

I had two Boker razors nearly identical but one was ordinary carbon steel and the other was silver steel.

The silver steel is lighter in colour and has a better mirror finish.

The blade is stiffer to use.

The blade gives for me at least a better closer shave but not a better smoother shave.

The blade holds its edge sharpness for longer.

I preferred the silver steel and to be honest I would now pay more for it, knowing I like it.

The other observation is that having owned a silver steel blade, I have seen others of a similar brightness and quality but they didn't get a specific branding. I'm sure they are silver steel as well.