I'm sure you're right Richard - and I'm very pleasantly impressed there's a part no. for something that wasn't even a BSA part. Just goes to show how good the service we get has become.
Stainless is a case in point of course - if one wants hex-head fasteners with nice knurling on and all that, then ordering up is the only easy way.
But you open up, as so often, a fascinating topic . . . .
At the more prosaic end of the market - what's wrong with using a screw or bolt (or anything else) as the basis for 'jury-rigging' a part? A bit of (in this case) 1/4 inch stock is . . . a bit of 1/4 inch stock . . . . and can be made into anything you like as long as it doesn't need to be . . . fatter than 1/4 inch! Compared with some of the crappy pattern bits out there, a decent home workshop job is often, I venture to say, better. For example, as to correct chamfers, better finish, exact length required, etc. And as to 'time saved' . . . time waiting for the post/mail man to bring (or sadly sometimes not to bring) things is very slow-moving for those of us who want our toys to work all the time.
Like a lot of folk, I haven't bought a nut, bolt, or stud in years, except for con-rod and cylinder head fastenings etc. It's a large part of the fun on rainy days, and the ability to do it has, for me, transformed classic vehicle ownership, along with making a million other things in daily life a heck of a lot easier.
Certainly can't claim to make all my standard spec nuts and bolts, but only because I still have a large accumulated stock of fasteners acquired over the years. I would make them though, every single one as needed, if I didn't have that stockpile. I suspect a fair few of us do?