Backshell for a Commodore Datasette connector. For each connector you need two (identical) parts and four M2.5x12 countersunk screws and M2.5 nuts. The hole is designed for a 3mm diameter cable. I used super glue (on one half) as strain relief.

What you see inside is a SN75176 to convert the line out of my MacBook to TTL levels. I included a schematic for the converter in the "Thing Files".