Bill & others;
I hope this post works. I have tried to apply a little colour to the inside stamps with a pin but it is a little messy since the stamps are so small. Hope you can see the details well enough to make out the left most mark that is in question. It looks like three parts with the middle connecting part missing.
Any ideas what it is? You may have to increase image size for better detail.
Regards;
http://www3.sympatico.ca/robertwiggi..._sideplate.JPG