My 2c - I ordered a heap of 300M 26 spline axles from him years ago. They were very poorly designed (shaft diameter larger than the base of the splines) and I started suffering failures. I've now run out of spares from the batch and I've been cutting up the old axles with my falling bandsaw to make drifts, trophies etc. I've just bought a Nitro cut to length 4340 axle and when I tried to cut it to length, my bandsaw can't touch it.
Surface hardness for heat treated 300M and 4340 is basically the same (over 50HRC) so that implies to me the heat treatment was poor or most likely non-existent on the 300M. If the business was still trading I'd take the time to do surface and through hardness on these 300M axles and spark spectro to confirm whether the 300M I got was actually 300M at all, because in service it's been nowhere near as strong as the EN-26 and Hy-Tuf (AMS 6425) axles we've also run.