It specifically fits the Milwaukee 2 & 3 Drawer units (48-22-8443 & 48-22-8442).

Custom modular battery trays that allow you to organise your batteries alongside other modular drawer inserts and build a flexible drawer system.

These have only been tested with the 5Ah battery models so that's what they were designed for.

Features – Flexible Design

Maximum Storage Space – Each tray takes up half of a vertical third and can fit two batteries apart from the 2Ah M12 that allows six!
2 & 3 Drawer Support – The inserts have been designed to be compatible with both the 2 and 3 Drawer Packout units. They will be most effective when used in the 3 Drawer units as that is what the depth of the insert has been based on, although they will still fit the 2 Drawer if you’d prefer