Well. I think fans first heard the nickname around the time McCartney released Give My Regards To Broad Street it was reported that when his name came up in the script, he wrote "Macca" instead of his full name. Also John used to call him "Macca" way back in 1965, there is an instance of it on the Beatles Anthology. In Ireland/UK anybody with a Mc in their surname is often referred to as Macca including myself. You can hear it here. https://www.youtube.com/watch?v=hyVBzI3evMM