I don't agree with rvannatta above. I use several old Pentax flash units, an FA160sa and a AF200t both are designed to work with the old Pentax cameras from the 70's & 80's. With the Camera set at P They will select your aperture on old K-"A" and new Pentax DA, DAL etc... lenses. let the flash select shutter speed or set at anything below 180. The flash will work with what ever ISO you have set and still select the correct aperture but I like to set ISO in the 100-400 range,
rvannatta is correct it will not work as P-TTL but the flash will select the correct amount of light based on a sensor internal to the flash.
For manual lenses set the ISO you want then set aperture based on the chart on the flash back the flash will select the correct light output for the image. So what I am saying any old auto flash will work for auto light output.
Just watch the trigger voltage on old flash units . see chart
Photo Strobe Trigger Voltages
Hans