M3M System: https://www.printables.com/@CowaBumble_4344087/collections/3209593

AMPS (attachment mounting pattern standard): https://www.bulletpointmountingsolutions.com/pages/amps-standard-patterns-explained

M3M: 10x10mm hole spacing, used with M3 hardware, holes sized for M3 heat-set inserts.

AMPS: 30x38mm hole spacing, I am using 6mm holes as it is close to the most common AMPS hole diameters.

To connect extensions/hinge plates: use M3M rail connectors