I thought I could because I saw a place to fit a key, but I'm not sure
now.

Water pipe comes up vertically and fits to a piece which on it's
opposite end (from the water pipe inlet) looks to be something like a
nut. At a right angle to this piece the radiator is connected, but
before the water enters the radiator proper, there is some kind of valve
that you operate using a key. Directly below the key I can see that
there is an opening.

Now it looks to me that turning this valve with a key might simply allow
water to pass out of that hole below the valve. IOW, I'm not sure this
valve actually turns off the radiator, but just drains it.

Anyway, does anyone know what that nut is for? If I screw that nut will
I then be able to close off the radiator. I'm reluctant to try it yet,
just in case water starts coming out if I mess with the nut. TIA.