Dan,

I do not know the direct answer for your question but consider the
following:

You must swing the compass and prepare a compass card.

You must adjust the compass with a "Diddle stick" a plastic on non-ferrous
object.  The adjustments consider the metal in and around the compass.
There is perhaps much more iron and steel in the instrument panel area with
throttle and mixture cables, some handles for fuel and dump valves etc.
This action is important in "Rag and Tube aircraft where there is a much
greater amount of iron and steel in the structure.

Considering our craft are composites the amount of steel in blind nuts seems
insignificant.

Bob Smiley