Trying to identify part name and correct bolt size my father-in-law sent us a bike, however one part was lost in transit. This is an image found online, but shows the part(s) in question.

What is the name of the toothed plate underneath the handlebars that secures them at a given angle? I originally thought that part was lost, but found it loose inside the shipping box. The bolt to tighten the plate, however, is still missing. I found a bolt that I thought would work, but the plate is still loose despite my tightening the bolt I have as much as I can.
So, and I'm hoping there is a standard here, what is the correct bolt size and length for that plate? In the event that I'd need to, is there an online source you'd recommend for buying replacement &quot;locking plates and bolts&quot;?
Thanks for your help.
I disassembled two that were readily available, one on a Giant and one on a Specialized. Both use an M8 x 18mm. The 18mm  length dimension did not include the head of the bolt. The total length was 24mm. I also noted both bolt heads were slightly shorter than industrial standard. Visually it looks like a bolt with a taller head would work but  1/2 head would  be exposed. Most small components of sub-assemblies aren't readily available the entire part would need replacement.  Although sometimes a local sop might have something in a stray parts bin. In your case since you only lack a bolt, any local supplier should be able to supply something that will work.