How to identify/find replacement bolt for STI I have an old road bike with Ultegra ST-6501 STI levers.  In the photo below you can see that the right lever has lost the rear bolt that holds the "outer" (shift up) portion of the lever in place when the "inner" (shift down) portion of the lever is thrown.



The absence of the bolt makes it difficult to shift down - the outer portion of the lever has to be held in place while throwing the inner portion of the lever.

The bolt must have fallen out in the middle of a ride.  I've tried to find a replacement bolt, but haven't been able to even identify the name of the bolt or a reliable way to describe what it is.  The documentation for the ST-6501 levers doesn't seem to mention it:

https://si.shimano.com/pdfs/si/SI-6C80D-001-ENG.pdf

I could use help identifying and/or finding a replacement bolt to get this lever working again.
You should be able to source a replacement many online industrial parts suppliers. You may not get an exact replacement, but you should be able to get a bolt that works. 

Presumably there is a identical bolt on the other shifter. You can use that to find the diameter and length you need. It will be a metric size. I believe that small metric bolts only come in 'standard' pitch. You'll want to get a bolt in stainless steel for corrosion resistance.

In the US McMaster Carr is a well known parts supplier. https://www.mcmaster.com/stainless-steel-bolts