My take on this, is that in 1925, the 8 bore wasn't that far out of the realm of 'modern'. Such a large gun gives a much more broad canvas to show one's expertise. the same size gun would have been laughed at, if in a typical 'modern' round of the day(1925).
: I do not believe this gun was made for any particular customer - merely 'look what I did' - or 'THIS is what I can do'!