Reproduction Sega ST-V CN5 terminator dongle. Per the official Sega ST-V (Sega Titan Video Game System) service manual, the system "does not function if CN5 is not connected with the Board (No. 839-0724)."
https://www.tindie.com/products/everten/st-v-cn5-terminator-dongle-sega/
https://www.tindie.com/products/everten/st-v-cn5-terminator-dongle-sega/