The 'Classic' 737 -3/4/500 have a database in the IRS which adds 'variation' to 'true' to drive the displays. Whilst the NG (more modern) table is pretty much correct, the 'classic' tables do not seem to have been updated since around 1980 and produce an error of +3.5 degrees in the UK. The FMC variation (used for nav displays etc) is correct as it is updated on a fixed 14/28 day cycle.
This is noticeable when established on a VOR radial when the drift line will be 3.5 degrees to the right of the radial. It also explains why the 'runway heading' never points down the runway(!). That is how I noticed it before anyone accuses me of being able to fly within 3.5 degrees of a heading :-)). I had to spend a while at one stage flying VOR tracks and noticed the 'oddity'.
As to working out what it does with an ILS localiser - my head starts to hurt, but I shall continue using it as an excuse!!!:-))