Serial numbers of a Benrus watch are usually located on the movable parts of the watch, or the working part containing the wheels and gears. Referred to as the movement serial number, it can be in different locations on the watch body.
Usually, one has to remove the back cover to see the movement serial number. There is also an external serial number on the watch case. Both numbers are different. For the watch serial number, use the movement serial number. Different companies could have manufactured the watch body and case independently, so the external case serial is not reliable.