It’s GFS, which is gauge plate. An easily harden-able carbon steel that’s normally produced to quite fine (manufacturer qualified) dimensional tolerances. Used for applications where it’s very hard wearing capabilities are necessary, cutting tools, fixtures, wear strips, precision packers etc. It isn’t hardened when you get it and therefore it is drill-able and machine-able but it’s not very ductile and I would imagine significantly more susceptible to fracture through vibration in an application such as a mudguard stay than mild steel. I’m sure you could make a GFS mudguard stay but it’s not really the right stuff for the job, I would say. Very tempting for many applications that it’s not really suitable for due to it’s nice precise finish, like using silver steel for wheel spindles.
A quick ebay search came up with this: 111544428342
Good luck!
