Hi Ade is right about the split rivets at the sides, your helmet is a Mk1 bridie as it has a rim, you will find that the earlier one was the "A" Patt helmet commonly known as a raw edge as it has no rim.
A little confusion here.
"Brodies", named after the inventor, British engineer John L. Brodie, are the raw-edged or also rimmed first two experimental versions. The name was passed on by some collectors to the official standard "Helmet, Steel, Mk. I" that was made from another steel alloy. Shapes of these three variants differed a little.
The helmet in question is quite interesting as it seems to be blueish in colour and shows some vertical dents in the dome that hint to a seperate unofficial production of another steel alloy.