FWIW:
While trying to find that elusive Suzuki bolt part #, I ran across a couple of related threads on the XS-650 Garage USA forum. Here's a snippet of a post made by grizld1 on 9/26/2008, almost exactly 11 years ago:
Mike's XS offers a straight-cut swingarm bolt to replace the undercut stock unit, which has a habit of breaking in aggressive riding. Dwayne Burge pointed out awhile ago that breakage at that point indicates that frame flex is taking place at the pivot and can be cured by welding a plate across the pressed steel frame members that support the swingarm pivots. If you ride aggressively enough for swingarm flex to be a problem, swingarm braces and braced swingarms are available from joewiseguy on ebay (or Google Wiseguys Choppers). 5twins suggested another fix, which is to use a TX750 swingarm, which is more robust and a bolt-on fit. I bought a used one to check it out; unfortunately it adds 2 lbs. of unsprung weight, which is a lot. Radian and FZ box-section swingarms have also been used; a search (go back a couple of years when you set the time parameters) will turn up several threads on these.
And here's a post made by 5twins on 4/18/2006:
What about the arm from an early GS1100? There's a bare frame in my dealer's boneyard and the swingarm looks very nice, boxed and aluminum I think. Would the pivot bolt be the one some guys replace the stock 650 one with?
12 years later, and still trying to get an answer on that swing arm bolt...