As far as I know, in 1996 Islamabad had no official plates. Officially issued plates started in 2004-2005. If the Sindh Police are bothering you then you need to tell them that in 1996 no official plates were issued then. Also get the present plates made in a non fancy way. Rectangular plates with a black background and white fonts of a legible size. If you have plates with fancy fonts and colourful background then you will definitely be stopped.