If you're asking about how to rack mount the SRX240 -- it doesn't require any special hardware. It ships with rack ears in the box.
The SRX220 is about 3" smaller (width) and uses a special bracket to fit in a standard 19" rack.
I just looked at the datasheet and there is a SKU for SRX240-RMK which I honestly find strange and I have to wonder why it's even there. Every SRX240 I've ever bought fit in the rack just fine with the provided ears that come in the box.