I have seen this exact set with plenty of 519 models. So it was sometime during the production of this model that the fastening collar ring of the hose and the blower coupler were manufactured of red plastic instead of plain metal.
Yes, according to Charles Lester's Kirby page the plastic blower coupler debuted with the model 519, replacing the previous red painted metal blower coupler.
It should be known that the 516's attachment box was similar to the 515's except for being brownish (rather than grayish) and bearing a plaid rectangular pattern... and also, of course, the red attachments.
I have my Grandmother's all original 519 and the box, logo, red plastic blower are the same. The coupler is metal. My attachment box sadly is water stained from a hot water heater that leaked.