While the volume knob of the DMP-Z1 is based on the RK501 it is a fully customised part requested by Sony and is not available as an OEM part from Alps, in fact with the collaboration the people at Alps wanted to use the know-how gained from the customisation in their own parts. And no it's not just brass plus gold plated, it's actually brass plated with copper and then plated with gold - a method Sony used while making the WM1Z which Alps didn't for the stock RK501. Also the Kimber Kable used in the Sony devices are more like Sony made with technology licensed from Kimber rather than straight from Kimber themselves.
https://www.sony.jp/feature/products/181101/?s_pid=jp_audio_product_201811_feature_1
EDIT: Also I must add that there's nothing exotic about the Chord Hugo TT2 hardware - the Chord magic lies in Rob Watt's D to A software, the FPGA chip used in Chord products are "off the shelf" just as well (a common Xilinx FPGA chip which anyone can purchase, but is useless as a DAC since, well, you don't have Rob Watt's source code). If you count custom D to A/signal resampling software as exotic, then Sony's own inhouse DSD remastering engine (remember, DSD is created by Sony) in the DMP-Z1 would qualify it as being "exotic" as well.
EDIT2: Also the knob on the outside of the device is just a cosmetic cap which is plastic with the gold plating - it's not actually the Alps knob which is fully housed inside the device. So you can't actually clean the actual RK501 custom volume knob FWIW.