This is the '4 Gill' design - a very distinctive longer profile with 4 angled-back gills on each side together with 2 'nostrils' at 10 and 2 o'clock. As well as the individual design, it has a unique construction - it's made of stainless steel that then receives a black nitrate coating. Some Tikka's use this thread size - if you have other calibres smaller or the same size as .308 (like 300 Win Mag) this brake will work for those calibres too.
recommend a screwdriver with a few wraps of duct tape around the shaft -
insert through the 'gills' of the brake and then twist on to crush the
washer and keep it all straight and tight. Just ensure that you don't
turn too far - if you back off you will need a new crush washer!