Interesting, unusual makers mark on pliers. What's It?

[BuddyC]

Hi All,
I picked up a rusty pair of pliers recently.  After a vinegar bath, it revealed an interesting mark that I've not been able to identify.  Wondering if anyone here knows.
IMG_6612 by William Clark, on Flickr

IMG_6611 by William Clark, on Flickr
     
Thanks!
BuddyC