SuperMontage (a/k/a SM) is an integrated trading system used on NASDAQ.

Test stock symbols

The SM system works behind the scenes but on rare occasions, it can be seen at work when a test symbol such as "ZVZZT" runs across the ticker. For a list of SM test symbols, refer to external references.

Reference

http://www.nasdaqtrader.com/Content/NewsAlerts/2002/headtraderalerts/SMStart2.pdf