Plus-X's published speed when the camera was made was probably 80. Kodak removed a 'safety factor' in their published film speeds and so Plus-X became ASA 125, though the film didn't change.
The Plus-X 80 card specifies EV 15 for bright sun. So, maybe the card reflected the true film speed without a safety factor. EV15 is 1/125 @ f/16 = 'sunny 16' for ASA 125.
The shutter speed is 1/50th of a second, given EV9.5 for for its maximum f3.9 aperture lens and the formula EV = log2 (f^2/t), where t is in seconds.