Welcome aboard.
Actually, It isn't just a crap shoot. Any gearbox attachment will work correctly with any Stihl commercial unit whether KM or FS model. They used to have the gearbox attachments in their catalogs, both online and hard copy on their own pages just for std trimmers etc. They all came without the short extension shaft like the KM would require for a long time, but eventually Stihl decided it was cheaper for them to just stock the KM units and let the buyer store or sell the short shaft as they saw fit.
I have bought all my attachments for my FS90 in both configurations- with or without the short shaft. They will all fit on the FS240/250 as well but are not listed for them as they create enough power and torque to damage them if a person without enough experience used them and didn't know when to back off with them on those two models, and that came straight to me from a Stihl engineer, not just guessing on my part.
IHTH