North America 1972-73:
North America 1974: More information on this system can be found here
North America 1975-on:
Described as a 'time delay unit' in diagrams but 'timer unit' in the Parts Catalogue (I'm sure I've seen it described as 'a limited period warning' somewhere but can't find it now). According to Clausager it will "flash if the ignition/starter was operated without the driver being buckled up", however John Keane on the MGOC forum says his flashes all the time when the seat belt is not fastened. 'Ignition/starter' is confusing - if just turning on the ignition without the belt being fastened sets it off it wouldn't need the input from the starter circuit. Also I can't see a need for it being powered from the 'always live' purple circuit, it only needs to be from an ignition circuit. However that may be a carry-over from the North American system that needs the purple supply for the 'key in, door open' warning function. There is an ignition connection, but only for the warning lamp itself.
Belt switches (UK 77-on at least) have a normally-closed connection with the seat-belt unfastened and an open connection when fastened.