Thanks for the link to McMaster Carr, but if they carry, say, 6mm SS flat washers with the same OD as the head of a socket head cap screw, I can't find them. If you know where they are, can you post a link?

In the small sample of 4 bolts I checked, 2 of them were 7mm, and one of the 6mm had a reduced head diameter. The flat washers used were all non-standard.

For me, the lesson from what seemed like a simple question from the OP is that much, if not most, of stem hardware is non-standard in some respect. Judging by what I found, trying to replace OEM hardware with off the shelf SS hardware is probably not going to be straight forward.