I had an Insurance card and had to buy a stamp from the post office every week to stick on the card. Then the employers started taking the N.I. money from our wage . You had to have a certain number of stamps on your card ( think it was about 18 months stamps ) before you could draw any benefits.
Maybe you are thinking of a P45(? ) or was it P(46)card that you were given when you left a job . This told your new employer your earnings, tax code etc. Then you could be put on the appropriate tax.