Ted, although it will fit on 1 of the rare Leica Thread Mount (LTM) Sonnars, it was most likely intended for use on a 50/1.5 or 50/2 Sonnar in Contax RF mount. All of the Zeiss 50mm lenses for the Contax RFs (as well as the Soviet Kiev Contax copies) take 42mm slip-on accessories (& usually 40.5mm threaded accessories). From your description, it sounds like your hood was made by a 3rd-party manufacturer either before or after WWII. Zeiss Ikon didn't mark their hoods "Sonnar" because the same 42mm hood was used for both the 50mm Tessars as well as the Sonnars.
FYI, the "official" post-WWII W. German 42mm slip-on hoods for 50mm lenses are silver matte aluminum on the outside (matte black on the inside) & always marked along the outside rim w/a product code of "1115 A42" as well as the words "Germany" & "Zeiss Ikon Stuttgart." The pre-WWII 42mm slip-on hoods for the 50mm lenses I've seen are brass, painted shiny black on the outside (inside is black finished but ridged to make it less reflective) are marked simply "42" & "Germany" on the outside rim.
Originally Posted by tedwhite
This is an aluminum lens hood (flat black on the inside) with a stainless steel ring on the lens end. The ring has three splits in it so it expand over the outside diameter of a lens. As it says "Sonnar 42" and it's old enough appearing, I suspect it was made for a Sonnar lens in M39. But, as I know virtually nothing about Leica stuff this is a mere guess.
I put it over on FS 35 classifieds, if anyone is interested. Actually, I'm more interested in how old it is and what lens it was made for.