Curiously to some, the British motorcycle industry used many different threads, shapes and unique sizes on their motorcycle nuts, bolts, screws and washers over the years. Learning the peculiarities of your motorcycle's fastening hardware may seem tedious, but having the motorcycle "correct" has become an art and a passion for collectors. It is accurate to say that different makers used shapes unique to them. Not all, but some did. Take Triumph for example: they typically used mudguard bolts with a slight dome to the head. Vincent, on the other hand, did not use these, but preferred to use a shape best described as extremely flat on the bottom side and with a flat top except for a slight bevel.
Washers tell a lot about the construction of a motorcycle. They should not extend but a small bit beyond the bolt head. The thickness of the washer is a variable that should be considered. So, if a bike shows big washers, they might work, but they probably aren't right; when someone knowledgeable examines your bike, it is certain to be noticed.
The material of fasteners is likewise to be considered. Most factories in Britain used inexpensive plating for fasteners rather than chrome plating, for the most part. Cadmium came into play before the war and was a cheap and pretty solution -- except that it began to rust in a few years, even in ideal conditions.

Today, stainless steel is in wide use in restorations for a couple of reasons. First, it never corrodes or discolors, as do cadmium-plated fasteners. Secondly, many fastener makers only produce in stainless, having given up carbon steel items. Caution using stainless should be exercised as these can fuse together under some circumstances. In some applications, this can mean you should use an anti-seize agent to prevent this. Stainless is much more expensive than plated carbon steel, but finding original fasteners to replate is now increasingly difficult.
These items, whatever their material and contour, must go on the right way. GENERALLY, that means bolt heads to the outside, securing nuts toward the inside of the machine. Use washers under the bolt head and the nut if possible and if only one washer is called for, it goes under the nut.

Avoid using replacement Allen head fasteners for the reason that they don't belong and they send a negative message about the integrity of the rest of the motorcycle. Allen head fasteners belong on Italian bikes and later German ones. Conversion of fasteners to metric is to be avoided if at all possible even though you might think no one sees it. A bad restorer of British bikes I once knew would go to an American hardware store, buy American threaded bolts and screws and run them through British thread dies. In the process, what was left of the threads was seriously compromised for strength. Take nothing for granted here.
"Cheesehead" screws are original to many old British bikes and certainly to Vincents. These are perfectly cylindrical heads cut for a flat blade screwdriver. Phillips head screws were not used on British motorcycles before and immediately after WW II.

Finally, examine how much threaded bolt, screw or stud extends beyond the nut top. As originally fitted, little if any extra threaded material stuck out. Sloppy assembly is the cause of too-long fasteners fouling motorcycles.
"The Devil is in the details" is certainly the case here. Lesser restorations will be spotted if you look closely at everything, including the smallest bits, the fasteners.