ElitePilot, would you care to share what "unusual" code that is? I have looked around but can't seem to locate it.
The other option is of course to look at green dot value and work out the calculation in reverse (GD-80 then divide by 2 for IAE, use 85 for CFM) and needs to be corrected for altitude above 20,000' (see QRH speeds table for details).
Of course the problem is, when in cruise, GD is rarely visible on the speed tape.