Actually, I believe the Memorex firmware (2.AD) is the latest and greatest NEC 3500A firmware available.
Very good media support and compatibility.
The LiteOn 1673S based drive will show up with a different drive ID string.
Usually you can tell the difference beforehand by looking at the product picture on the box.
The LiteOn version will show a drive that has a volume knob and headphone jack located on the front of the drive. The NEC version has no volume knob or headphone jack.