A great bit of free advertising that the customer paid for (unless you could persuade the salesman to throw one in for free with your new bike). These musettes or "bonk" bags came in a variety of designs - few turn up though these days - consigned to cycling history as far as Rattray's is concerned but still used during the grand tours (Giro, Vuelta, TdF et al..) to this day.....

These ones probably date from the early 70s. Ideal for your banana, slice of fruitcake and your sannies of course. (and for swinging round off your back when you least expect it)!