What do You Make Of This ??

[ViggoG]

I found this strange mark on the underside of my 1906 AE 55xxx.
It is located directly under the chamber next to the retaining lug directly above the "5".





I have made a few tentative inquiries with the only response being :
" I've never seen anything like that before".
Any IDEAS or SOLID INFORMATION will be greatly appreciated !
ViggoG
PS: Ihave found my Macro Lens and now can show the Strange Mark greatly enlarged below.