A stringer? Isn't that what you hang a plumb off?
One of my EASA pet hates is the messing about with licence groups, the CAA one based on AUW's was so simple, the current one which changes everytime a new model comes out is not only a bureaucratic nightmare, but pointless you now can have licences with aircraft that have moved into other groups or are not listed, and they will only be corrected at the next licence change. I often wonder the legal ramifications if you certify an aircraft listed on your valid licence, but has been moved to another group.... I thought it would settle down, but then they brought out the new light licence. Simply doing it on weight got around all this crap. The guys I felt sorry for were those that paid to get a EASA licence to cover their DC3 etc, for it to be removed off it a year later and back onto a section L.