Again, thanks for everyone’s comments.
Hi Sean, I don’t really know the grade of most of the aluminium I used.
I got it from various suppliers on eBay, and was just described as aluminium plate or tube.
I do know that the 3mm side plates are 1050 grade.
I didn’t notice any real difference in hardness of the various pieces I used, it was all relatively soft and easy to cut and file.