I just got thru rebuilding a set of 059a Keihins on my '75 550. If you're looking for replacement stainless slide assy screws, top cover and float bowl socket head cap screws, go to Microfasteners.com. They have 3X10 button head socket screws for the slide assy and the 4mm and 5mm for the others. You have to order a minimum of 20 each for anything, but it's cheaper than anyone else wants for their kits. And, this is the only place I found the 3X10's for the slide.
Also, I found that the parts fische on bikebandit calls for 4X12 float bowl screws. These carbs actually use 4X10's if you don't use lock washers. The 12's will work, especially with washers, but they're longer than needed.
Hope this helps.