Kpt. Otto,
I may be able to clear the situation up a little bit.
Neither the stock version nor GWX uses an OBS_periscope.tga. The [G2B I130] block in the menu_1024_768.ini file that you mention, has always pointed to Periscope.tga I believe this is because stock and GWX use the same background for both the (F3) Attack periscope and (O) Observation periscope screens.
In Ducimus' "Periscope Stand Alone Mod", no Periscope.tga or OBS_periscope.tga files were included, BUT the "menu_1024_768" file has the change in the [G2B I130] block to point to a "OBS_periscope.tga file. So if you have only applied the "Periscope standalone" mod you may have the problem with the Observation scope. Lots of people don't use the Observation scope, so I don't think too many people even notice the problem, if they have it.
In Ducimus full "Flavored to Taste Mod", he provides both a new low-contrast Periscope.tga and OBS_periscope.tga file (Although these files are really identical and OBS_periscope.tga is just a copy of Periscope.tga that has been renamed). Therefore, you will not have a problem with the observation scope.
This is not remotely a criticism of Ducimus' mod, which I absolutely love.
I suspect (and obviously I'm just guessing here) that at one point, changes for a mod that was altering the Observation Scope to have a little different background from the Attack Scope got carried over to these mods somewhere along the line. Given the sweeping changes in Ducimus' mod I am amazed there aren't any other inconsistencies like this one.