Thanks Caryn - the construction is still looking early '70s to me but I'm thrown by that ruffle trim. It looks like the sort of fancy stitch a home sewer would do or perhaps even a little hand crochet, which seems unlikely. The colour is at odds with the rest of the dress too. Also - more importantly - it looks like it's not finished off properly, like the hem has just been folded over with the raw edge underneath and then the fancy stitch applied to the fold. This does not look consistent with a professional dressmaker at any time so either a) it's home sewn and more modern, perhaps even early '80s as you suggest Caryn, made by an older person who has more traditional methods - or it's an update and addition.
Dee Dee, the colour is more vibrant on the ruffle so rather than it being from a repurposed part of the dress, I wonder if it was an unused scrap. Also, you haven't mentioned the arm openings.