Vintage Sonor tension rods are 1/4"-20.
I need to ask are you wanting to ditch the flat sided screws? The only thing that will work is the sonor slotted. You could replace the insert but then you are looking at more money. On all my phonics I kept the screws and just removed the little clip from the lug insert. I have about 13 rosewood phonics and most of them have them.
If you are looking for new then the best place is at that ebay user samiam(not related to him at all, but have bought many hardware pieces such as tension rods...). He has a few different lengths in stock. I'm guessing yours is about 46mm long.
Other sonor dealers can get you stuff too, in fact one of the best is in Germany-ST drums. Very knowledgeable about sonor. They also have a fair amount of vintage stuff. I just bought some things last month from him, shipping is the only downfall... When ordering it must be specified 1/4-20 because the modern sonor is 12-24 just like other drum companies.
Good luck, Gary.
Gary