A lot of newer devices nowadays have a 3.5mm jack for MIDI-Out. Unfortunately – unlike the regular DIN5 plugs – there is no standard in this type of MIDI-Out. You could say there is a polarity issue. One manufacturer uses polarity A, the other uses B.
But – you can plug this adapter to a 3.5mm mini jack stereo audio cable to get a DIN5 MIDI connector on one end. There’s a switch on this adapter so you can match it to any mini-jack-MIDI device you connect.
Also – if you use this adapter on the RK-004 DIN5 out and set the RK-004’s port to ‘sync mode’ you can drive multiple devices which run on sync. For example multiple Pocket Operators, Monotribes, …