This looks like the "goggles" for my dual range Summicron, including the mount (not a dove tail). Mine too is engraved E. Leitz Wetzlar. A 1972 Leica catalogue I have shows the DR Summicron "Goggles" simply engraved Leitz Wetzlar.
The notch and the divot are part of the "system design" (for want of a better term) that keeps one from using the close-range without the "goggles" in place (or the regular focussing range with the "goggles" in place).
David