This Module is currently available.
3,5/6,3/Phono Adapter
This passive module is a handy converter between different connector formats: 6.35mm jack (1/4"), 3.5mm jack (1/8"), and phono (alias RCA or cinch) at channels 3 and 4.
These merchants probably sell this module. Huh?
Analogue Systems RS-270 Adapter