When winding curtains move left to right, upon release they move right to left.
Being Spotmatics are manual cameras open the aperture to f2 or remove the lens, set the shutter to 1 second, cock the shutter, open the back, point the camera at an evenly lit light colored wall. watch the second hand of a clock and trip the shutter as the second hand reaches a second/minute mark. Observe that the second curtain closes as the second hand reaches the next second mark.
Tolerance is .8 to 1.2 seconds, about the width of the second hand either side of the next mark on most clocks/watches.
Light leaks will often extend into the rebate area and between frames, extra exposure limited to the the image frame is shutter or light leak from the viewfinder or mirror. Strong side lighting coming into the viewfinder eyepiece can cause light flares in the image frame. The only way to eliminate this type of light leak is to block the light source from the viewfinder eyepeice.