These staples were a common method of fixing cables to a flat surface. They were placed over the cable and hammered into position.
The insulated plastic part is designed to protect the cable from the metal staple.
This is a box of white staples, which for some reason also includes two brown ones.
The box still has the price on - 6d.
Probably from the 1960s, and certainly before 1971 due to the pre-decimal pricing.
The box from two different angles.
Size 5/8 inch, Made in England.