Looking at the question in another way re any views on the different costs...
They are 'only' bolts aren't they?! So a less than scrupulous trader could easily approach one of thousands of suppliers, probably in low cost countries, probably who have no knowledge at all about Norton's or old motorbikes with their needs and foibles. They'll give the supplier an approximate spec that will 'probably be OK for most users most of the time' and get a huge batch made for peanuts. Its then SO EASY these days to flood the market via autojumbles, on line auction sites, and less reputable dealers.
IF these sub optimal parts lead to engine failures... they are simply creating a demand for yet more pattern parts... therefore fuelling the cycle... and creating good future business...!!
Alternatively, a genuinely scrupulous trader would want to specify the correct steel, the correct manufacturing processes, the correct specifications and tolerances and finally the necessary quality assurance measures. They would then have automatically narrowed down the potential suppliers to a few, specialist outfits. This takes a lot more time, effort and money. And will lead to lower sales as a result of being 'expensive'. Often the only 'thanks' they get for this approach is to be labelled 'rip off merchants' !!
Clearly, the difference in cost price between the above scenarios is beyond compare. But only one of them ensures the correct quality!
The real issue, IMHO, is the willingness of the masses to buy such important components from unknown, faceless vendors on ebay / from autojumbles, etc. Basic, core, critical mechanical components should ALWAYS be bought from reputable sources. Anything else is simply Russian roulette !!
That's why I only by from a few vendors, repeatedly. I want parts that will fit and function as intended, with minimal fuss and failure.
All only IMHO of course.