I was told by a Militaria Collector that M&C stood for Military and Civilian.
Alex Nichols book page 337 shows a 1958 letter from Reuben Craddock and Sons which mentions Ministry of Supply, and Crown Agents. I prefer to agree with your current thinking even though all theories are possibly correct depending on what time period we are talking about because the Ministries changed names.
Now, just to throw something else into the mix. I have a broad arrow marked cuff with a capital i under it. It could stand for India, but it could also be Ministry of the Interior. see Wikipedia link
Could the M.C.I. marked handcuffs be Military, Civil, Interior Departments? or do I think too much - or not enough?
"An interior ministry (sometimes ministry of home affairs) is a government ministry typically responsible for policing, national security, and immigration matters. The ministry is often headed by a minister of the interior or minister of home affairs. In some countries, matters relating to the maintenance of law and order and the administration of justice are the responsibility of a separate justice ministry."
Joe, I believe you named the Hiatt "Home Office" cuff after these departments which were once called the Home Office. Maybe the fluffy ones can be called "home affairs"