Hi Christian,
Gotta say I agree with Bonnar Bloke. There are some v suspicious marks from the lower holes in the photo. I'd be paint stripping the area rather than sanding as sanding can mask fine cracks. Then a good look with a 10x glass.
Reel mowers are not my area of expertise but finding and repairing fatigue cracking in structure is something do a fair bit of.
Hope you enjoy your project.
