>>4293765A newer pentack branded one will probably be the best, but none will be good.
https://www.pentaxforums.com/lensreviews/Pentax-Rear-Teleconverter-A-2x-L.htmlis probably your best choice, if your lense is on the list.
This is not a film pic, but a noname 2x on the back of a canon 135/2.8, had to stop down quite a bit to get usable sharpness, and that's with a short lense on an e-shutter digi.
On a film body with a physical mirror and shutter, it will almost be impossible to achieve sharp shots, even if you can get focus and enough light for a shutter speed, and even then the IQ will suck.
