This Module is currently available.
Headphone Monitoring Amplifier 500 Series module
500 Series double-slot module headphone monitoring amplifier with Phonitor Matrix for two headphones.
https://spl.info/en/products/3338/hpm/overview.html
These merchants probably sell this module. Huh?
Included in 30 Racks, 17 are public:
SPL HPm
SPL HPm 500-Series Headphone Amplifier
SPL HPm 500 Series Headphone Monitoring
SPL HPm