Module is available as a DIY project only.
No info about availability.
https://www.hairballaudio.com/catalog/fet-500/fet500-revision-f
Included in 12 Racks, 5 are public:
Hairball Audio FET/500 Rev. F 500
Hairball Audio FET/500 Rev. F 500
Hairball Audio 1176 Rev A/D/F - FET 500