max voltage for Electrocraft servo

I have some used Electrocraft E543 brush servos. I did track some
specs on them, but there was nothing on max. motor voltage.

Does anyone know what this is? Or is there a way to determine this
from the other specs (KT, KE, max continuous current, motor terminal
resistance at 4 amps, etc.)?

I know someone who is running these at 67 volts without any problems.
But I am cautious (as all this servo stuff is new to me) and the
Geckodrive 320 (which I haven't tried yet becuase of this question)
information states not to exceed the motor's rated voltage by 5 volts.

Grant