This is a Grade 5 Titanium 17mm bolt in M12x1.25 thread pitch for B / D  Series Engine Mounts in 100mm length. These are used to fasten the mounts into the brackets on the chassis. These work on all 3 mounts and are available as such or individually. 
Some aftermarket mounts ask for 1 bolt in 120mm length measure to make sure you’re getting what you need. This is 100mm not 120mm.