Talk about thread drift!
Back to the original question
Later ISIS indicators have what is called a "boresighting" function. This uses ADIRU attitude inputs to accurately match the ISIS attitude to that of the ADIRU's to minimise errors.
When installing a new "boresighting" ISIS the boresighting function needs to be used to, in effect, calibrate the display. It's a sort of once only caging of the gyro.
Remember that a data bus input to another device contains all the info on that bus. The receiving unit extracts the info it requires in order to function. Earlier units extracted only heading. These later units also take attitude data.
Hope this helps.