A Captain and First Officer of a CE-560XL report on their write-up for a compass deviation card entrees being faded and difficult to read. The MEL was applicable for a GH3000 with a standby attitude indicator. They had a Meggitt unit which made the MEL invalid.

Narrative

We wrote up an MEL for the compass deviation card being illegible. The MEL was applicable with a GH3000 standby attitude indicator. We had a Meggitt which made the MEL invalid. Install all aircraft with the same instrument. Make it easy to determine if we are flying with a GH3000 or a Meggitt (they look identical). Supplemental information from ACN 824864: I was notified on Feb/XH/09 by the Steward on duty that on Feb/XA/09 MEL 18-679 was misapplied to aircraft. I was the Second in Command assigned to Aircraft X and the aircraft was at ZZZ. I was briefed by the Captain that the compass card entries were very hard to read because they had faded. After the Captain reported the discrepancy to the company; he told me the discrepancy could be deferred. I looked in the MEL and asked the Captain if we had the GH-3000. He said we did; and then I continued my Second in Command duties. Change the wording in the MEL from 'GH-3000' to 'GH-3000 Standby Flight display.' Callback conversation with Reporter ACN 824865 revealed the following information: Reporter stated he had confused the Meggitt unit in their aircraft with their other CE560's that do have the GH3000 standby system available and allow for deferral of their primary compass. The Meggitt unit only has an attitude and heading. There is a whiskey compass. Reporter stated he has since realized he can tell the difference between the two units. The incorrect deferral was found during a maintenance audit.