Sorry to come in late on this one! I primarily restore fountain pens, and the same plastic was used on Parker “51”s as your cowling. Lighter fluid (Ronsonol), is an excellent option for this one. The naptha will dissolve any old adhesive almost instantly, but is just fine on that tough old acrylic. You should just be able to wipe away old sticker residue with a cloth, rather than having to resort to any scrapers, however benign.