The heads have been removed from the Odemars German Officer and a tiny bit of guitar string inserted into the base of the head. About where the spinal column would be located. I use a Ernie Ball Custom Gauge 8 electric or acoustic guitar string. 80 cents plus tax and three feet long. You can transplant a lot of heads with one of these.
As you can see the middle gray figure has been given a new base. His original base was lost to history and I replaced it with a bit of sheet styrene tile from Evergreen Plastics. I love these tiled bits for making straight line cuts. Two tiles across and four tiles long and you can make zillions of nearly identical bases. This size gives a base about the same size as the traditional Airfix rectangular base, same thickness too.