These Metal Pet Tags are from UniSub. They are available in 3 different sizes and are printable on one side which is a rather bright white. Being UniSub the sublimated image transfer is fantastic.
Overall measurements of these are shown below and they are made of aluminium hence making them light to fit onto pet collars but robust enough to withstand day to day wear and tear. They also come complete with a clip to fix to any animal collar.
Shapes available are:
Bone: 38mm x 25mm x 1mm
Circle: 32mm x 38mm x 1mm
Heart: 32mm x 35mm x 1mm