Hi It is quite likely to be a rather generic cone tweeter. One that is about the right size and likely similar in performance is the GT25 by Goldwood. Parts Express and Madisound carry them. Last time I got a few they were under $5. A great buy. Get two and replace both. It may be a good idea to check the crossover capacitor. Usually in systems of that vintage it was just a single non-polar capacitor in series with the tweeter. It might be bad and not the tweeter. If you can find it, temporarily jumper it and see if the tweeter makes sound. Do this at low power levels just in case the tweeter is good and the capacitor is not. You don't want to damage a good tweeter. If the capacitor is bad just get a replacement from one of the parts houses. They are pretty cheap.
Good Listening, Bruce
DIY Custom Dual 701, Jelco, AT33PTG/II
Empire 598, Origin Live, DynaVector Karat23RS-MR
Sota, Jelco, AT-F7, Hana EL, Benz MC Gold Ruby Tip
Custom All Tube Electronics, Martin Logan ESLs