I think it unlikely that these bolts were " made in the machine shops of BSA itself". I lived and worked in the Midlands for a few years shortly after the demise of BSA and even then I was surprised and a little charmed by the existence of the traditional industries seemingly unique to each village. Wednesbury for locks, Dudley for chains, Bloxwich for metal stampings, engine blocks were cast in Wolverhampton, and nuts and bolts were made in Darleston. Each small industry provided the pieces for the large manufacturers. I worked in several, even the upholstery for Leyland Cars was made in small factories.
The demise of the large auto producers, both car and motor cycles, was devastating to not just the actual communities around the assembly plants.
I think it much more likely that the domed bolts, in all their array of threads, were bought in. In the glory days of the '60s, with little competition, the extra cost was nor particularly relevant. They were abandoned about the same time that using the "right" thread, was changed to using the "cheapest" thread.
I have copied the circular indentation to signify "unified" treads in a similar way, by putting USA-made bolts of the proper threads in a lathe, then placing an end mill in the tail stock and cutting the circular marking that way.
Birmingham was the workshop of the world, even in the 80's when I was working in the Auto industry there were hundreds of these multi spindle machines around. My first job was to convert a shop of 20 of these machines from HSS to Tungsten carbide tooling.
I believe the domed head was for both clean lines and less likelihood of a bolt head catching on a branch etc, a lot of off road riding at weekends in the UK. Just a guess.
perhaps the question is the wrong way round ...... why not domed heads on oriental and American bikes?
They used domed because that is what should be used if you have a bit of pride in the way you design bikes, and how they look. A pointless expense you say? well that depends on your perspective. They stopped using them when they had to compete with the souless bikes built to appease the gods of the balance sheet, where every cent has to be shaved, every dollar grabbed.
When rebuilding my bikes I also domed the heads of the stainless bolts using a rednecks lathe, i.e. a cordless drill and an angle grinder. Quick, simple though perhaps a tad hazardous.
I think there were a few reasons, if you are making your own in house fasteners, why not reduce the head size to save space and weight and materials. The better quality marques like Ariel made a point of having particularly well made fasteners. If you are starting with hex bar a reduced head Across the flats size saves a lot of machining, also allows for neater construction, many of the clearances around fastener heads are tight on a motorcycle, designers made stuff as tight as possible to look pleasing ,and save weight/ materials. Once you have done all this you might as well add a dome/ chamfer, thats what separates machinists from the animals.
The domed head is an artifact of the Machining process . its easier to "part" a piece without parallel sides . a straight Plunge parting tool is forming a Groove on 3 sides at once . ( you have to have your tooling more precisely placed to pull this off )
if you "part" as a decreasing wedge you can face the dome as the "parting " cut is made deeper and more narrow
the cutting can be done with a pointed tip and not a forming tip . The Parting cut only becomes a parallel cut right before the piece is ready to fall off anyway
I would say that domed bolts nuts are used for aesthetic reasons only.
None of the photos I have seen in this post show what I would call domed heads. They all look to me like they have a radius on a big chamfer, which looks nice but does not fir the proper description of a domed head.
When you haver access to tons of stainless fasteners but they are all metric it is rather frustrating when you really want BSF, or Cycle etc.
Still I have been known to delve in and rob some metric stainless fasteners for those areas where it doesn't really matter what thread is on them.
I always expect domed heads to look like the nut I picked out of the stores today and photgraphed.