As far as I can tell, the use of the word 'harpoon' to refer to 'harmonica' is pretty exclusive to 'Me and Bobby McGee' - I imagine the writer of the song, Kris Kristofferson, was engaging in wordplay rather than using a pre-existing established meaning - between the context and the prominent harmonica in his original version of the song, it's fairly clear that by 'harpoon' he means 'harmonica'.

When I say wordplay, it's likely specifically wordplay on 'harp', which is a common word often used to describe the harmonica in a 'blues' or 'country' kind of context (and in this kind of context, even before the 20th century, the harmonica was getting called the harp, the 'mouth harp', or the 'French harp' despite not resembling the stringed instruments we other usually refer to as 'harps').

As to the bandana, yes, the red bandana was symbolic long before the 1960s. According to a journal article by Margaret Poulos, 'El mundo es un pañuelo*: The Bandana as a Global Symbol of Resistance' it was originally Indian headwear (as in, the India in South Asia) before filtering through global trading networks connected to Britain. Poulos argues that the bandana is used in a variety of contexts, but is very often used to connote resistance of some sort; the red bandana specifically was already being used by unionist miners in America from the 19th century, especially to signal membership in the union when they it was necessary to fight government militia members sent to break strikes etc (who typically wore white neckerchiefs). So, yes, you may well have seen working-class people in trainyards in the mid-20th century wearing red bandanas.