RC Allen 2600, 26EK, 26AK & 26BK Optional Slip Indicator - Horizon Not IncludedAdd an Optional Slip Indicator to satisfy FAA AC91-75. The slip indicator, P/N 444-0010-01 can be used for all new RC Allen Attitude Indicators manufactured after March 4, 2011. All RCA2600 Digital Horizons use P/N 444-0010-01. For Electric and Vacuum Gyros manufactured before March 4, 2011, use P/N 444-0011-01. When ordering to retrofit an older instrument, refer to illustration.