You will find stems with this logo on a variety of bikes. While I can't give you any information about the manufacturer, they appear to make and supply stems on an OEM basis.
If you search ebay for "NL Quill Stem" you will find examples branded GT, Kalloy and others, in a variety of styles.
Usually they are supplied for a (large) bicycle manufacturer as the cheapest possible solution at a given quality to attach the handlebar to the fork, though you may find other aftermarket examples branded for large distributors that would like to have their own-name parts.