The NL-816 is a clear glass end-view symbol tube manufactured by National Electronics. It can display various symbols related to electronic units. The symbols have two columns that allows the combination of symbols to form units like "mV" or "GHz". It is likely that this tube was used in digital Multimeters. Also this is the only SK-136 Socket tube i know, that is mounted in a 90 degrees rotatet position. This tube is very rare.