I think the sectioned Mills is a No23 Mk1. It's essentially a No5 with the base plug beefed up to take a grenade launching rod. The No23 Mk2 is also virtually the same but the base plugs are a little different (& more variable from manufacturer to manufacturer) in shape. The No23 Mk3 is the extensively redesigned No36 body with the No23 Mk2 base plug fitted to allow the earlier Adams (sheet metal) 'launchers' to be used with the 'new' No36 shaped grenades.