I have four 45s, 3 US and 1 UK. All of the caps have "PARKER" stamped under the clip and "Made in US/UK" on the back. Two of them, 1 US and 1 UK, also have a large "45" stamped on each side between the front and back stamps.
Might this indicate, approximately, when they were made? There are no yr/qtr stamps on them, like on my Frontiers.