57 Unknown bolt marking
 

I removed the bolt from the lower rear of block passenger side that holds the sparkplug separator. Bolt has unusual markings...a C with an underscore on the right side top. Also attached by the bolt was a thin brass plate 1" x 2" with the following number stamped on it: 126814. I'm trying to add a photo but not sure I'll succeed.
Does anyone know anything about these?

Another photo of the odd bolt

Some engine rebuilders, mark their rebuilds similarly. It's usually a metal tag, with their name on it, with a serial number for the rebuild. Sometimes a date, also.