These are intended for use with CNC machines made from 2020 profile aluminium extrusion. Both enclosures fit three GX16 aviation connectors and one of them has space for an additional GX20 connector. They are held together, and to the rail, using t-slot drop in nuts and M4x25mm bolts. For more details see my blog: https://blog.discoverthat.co.uk/2019/04/2020-t-slot-gx16-enclosure.html