Pakistan changed over to the decimal system in 1960 and all earlier “Ana” stamps needed to be converted to “Paisa” denomination face value. Thus on 1 January 1961, a series of stamps bearing drawing of Khyber Pass and Shalamar Bagh were issued for the face value of 1, 5, 7,10,13,15,20,25,40 and 50 Paisa.
While I was a kid, we would see these stamps most commonly used on the envelopes that would be sent to us by our relatives. That is why I still have a fare quantity of these stamps as duplicates.
The definitive series depicting Khyber Pass and Shalimar Garden, was twice produced during 1961-62. But the postal authorities were not satisfied with the shape of Bengali inscription denoting PAKISTAN. The inscription was redrawn in new style and all paisa values were printed with PAKISTAN in Bengali accordingly altered. Distinction of the new font is a straight line over all the letters resembling PAKISTAN which also connects letters with each other. This beautiful set of stamps remained in use till 1970s.
Related Link: Stamps of Pakistan 1961-1980 (Pakistanpaedia)