One has to look really hard for cracks in places like that. The 172 has a similar cracking problem at the bottom of the aft doorposts. I have found several cracked. Cessna didn't do the standard practice of smoothing and radiusing punched of cut edges like that, so the rough edges are stress risers that eventually lead to cracking. The 210 has an AD against its wing spars for cracking, and I found similar roughness on the edges of rivet holes in the spar flanges. Again, no dressing as is usually practiced. It's the same reason why nicks are to be dressed out of a propeller's blades.
