Very common on older installations for securing flat twin & earth type cables.
Clips like this are still available from a few suppliers, but are rarely used as they take significantly longer to install than the plastic clips which are placed over the cable and then hammered in.
They do have the advantage of having a neater appearance, and are also fire resistant, unlike the plastic types.
This style of clip is secured to the wall with a single nail through the centre hole. The two halves are then folded over the cable, and the smaller tab passed through the slot in the larger one. The tab is then folded over to secure the two halves together.
It is important that they are used the right way up - the centre hole has sharp edges on one side, which should be placed against the wall so that the sharp edge does not damage the cable.