ARW wrote:

I wonder who is making the SPDs?

strangely they seem to be black rather than pale grey.

https://thomaselectricaldistributors.co.uk/wp-content/uploads/2020/10/H6820S-scaled.jpg

these look quite similar, not identical though

http://www.surge-protectiondevice.com/photo/pl24657634-type_2_pluggable_2p_modular_surge_arrestor_320v_40 ka_surge_protection_device.jpg