Strange marking on barrel extension
 

I was having a look at my 1917 all matching 1917 DWM and noticed a marking on the barrel extension. Along with the 4 standard proof marks that i have seen on many Lugers from this era on the Barrel extension near the breach end of the barrel, I noticed a 5th marking.

It looks like this:
.
/.


It is found to the right of the other 4 markings and is about a half line lower than the other 4.


Please keep in mind that is the best i can do on the computer. the top dot should be slightly to the left and down and makes to make it more symmetrical. Basically looks like a kind of per cent symbol or old styles division symbol.

Does anyone know what this is?