I have at hand evidence that JBG was singing the song before it was published in 1876. It was printed by the Glasgow Poet's Box in 1875. I also have a note that it was printed by Sanderson of Edinburgh who was printing right into the 1930s (and throughout the 19thc so that's not a lot of help)under the title 'Cockles and Mussels Alive, O' which could be the Yorkston version. I only have a title in their catalogue. One of them was also printed by Dundee Poet's Box, presumably the JBG song, but again this is only from a catalogue.
I'll check the GPB sheet to see if it gives further details. It will at least give a precise date and he sometimes gives a date for an earlier printing.