In civilian use, the "Automatic Variation Setting Control Unit" is today the MagVar tabular database in the non-volatile memories of the Flight Management Systems. It is next due to be updated in 2012, when it will only be two years out of date. (The current MagVar tables are about 11 years out of date). The separate GPIs used their own integral MagVar conversion cam to help them continuously plot Long/Lat long before the advent of Inertial Reference systems, but circuit flying tended to get them into a groove, if you see what I mean.
