The model 72 is a fountain pen in black — more rarely coloured — plastic with a gold-plated cap. No version of this number was made entirely in rolled gold; the all-gold counterpart within the same series is the model 82. A pen finished throughout in rolled gold whose cap ring bears the number 72 is therefore very probably an 82 with a swapped cap.
The fact that the cap sits neatly and that the barrel looks a little smaller than on a comparison pen is no argument against this: the fountain pens in this series were built in two sizes, which can be read off the second digit of the model number, while the ballpoint and the propelling pencil were offered in one size only. Caps from neighbouring sizes and finishes can therefore be interchanged, and such mixtures are common on used pens. A special run for foreign markets can never be ruled out entirely as an explanation, but it is unlikely here. For a collection the rule is plain: a made-up pen is worth considerably less than an original one whose barrel and cap belong together.