I have one of those axles for my NX5 RS250 which I'm pretty sure came from an RC45.
It is made from steel and does look a bit different to your axle.

For info below are the parts I bought from Spiral Spinner in Japan for my axle.

Right hand side RC45 aluminium rear wheel nut M46 x 1.5mm pitch large aluminium left hand thread (red only) @ 12,600 yen
Tachyon part number 5100-TW4B-000

Left hand side RC45 titanium axle nut M38 x 1.5 x T10 @ 29,000 yen
Tachyon part number 59040-RC45-000

Large R clips to suit the wheel side of the RC45 HRC axle @ 3500 yen.
Tachyon part number 52200-RC30-000

Hope this helps.