This one has been bugging me all day...

My goal is to change the thickness of the attached part.  In the part the material thickness is controlled by the thickness global variable.  So to change it I would normally go to the equation editor.  In the equation editor the thickness variable is greyed out and linked.  Further digging tells me the thickness is linked to the sheet-metal1 feature.  When I look there I cannot find the source dimension that the global variable is linked to nor is the usual "link to thickness" checkbox available in the feature.

Am I missing something obvious?  Or is this the result of a part originally made in 2006 finally being converted too many times?

Any help would be greatly appreciated.