FWIW the reflection in the cable has a pole at 242 MHz for a 36" cable and 0.678 velocity factor.
An interesting experiment would be to measure the output of the SG503 at the generator and at the end of the cable. The cable spec may have been chosen to compensate for roll off in the generator at the upper frequency limit or to prevent a bump in the response below 250 MHz which a longer cable would produce.
