fence support

I'm referring to these... Which way around is the 75 mm x 100 mm please? I.e. if the spur was bolted to a 3" x 3" wooden fence post, would it be exactly the same width or half an inch wider each side than the wooden post ?

Hello I'd like to erect a heavy trellis with 70mm wooden posts using concreted-in metal post supports. Please see the photo below. The trellis will be 7ft high and although it is quite heavy, since it is full of holes, it will not catch the wind in the same way as a regular fence. My question...