As a member of a Royal Marine family (grandfather, father and brother were all in the Corps) I am absolutely sure that I remember watching Royal Marine Bands before the match and at half time at Home Park many times over the years. In fact I would go further and say that they were regulars in my early years (I was first taken to a game by my father and uncle in, I think, 1946).
My father was so proud of having been a Royal Marine that, as they marched on and struck up "A Life on the Ocean Wave" he would always comment on how the Royal Marines had the best bands and were better at marching and drilling than any other British military unit - especially the Guards.
On the other hand I can't be absolutely sure which Royal Marine Bands came. Every RM base such as Plymouth, Portsmouth and Deal, had its own band and every naval ship visiting Plymouth would have been able to provide a band too - and sometimes bands amalgamated. But I can't believe that in our long history as a club we have never before had a visit from the band just down the road at Stonehouse Barracks - HM Royal Marines, Plymouth. If it was true it would be a disgrace!
My memory is that, as time went by, visits by military bands became fewer and fewer and their place was taken by local brass bands. I seem to remember that the Lostwithiel Silver Band came regularly. My Dad wasn't quite so impressed by them!