I've bought over a dozen STAR TREK plastic model kits over my life of American kiddiness.
As you know, plastic models from AMT or Aurora in the 1960s and 1970s nearly always came with a sheet of decals. You'd cut out the appropriate piece of design and soak it in water. Remember how the pieces would curl up when you dropped them into the water?
Then after a minute or two the design and its clear surroundings could be slid away from the backing paper and oh-so-delicately positioned onto the appropriate area of the model. Then you'd try and hold down one edge of the decal with a finger and use another finger to smooth the decal out and push out any air bubbles.
I'm pretty sure of the reason I have an extra sheet from the Enterprise kit. I was going to buy two Enterprise models and merge them together to make my own dreadnought, as seen in the Technical Manual:
At any rate, for some reason I still have these sheets from a "Space Ship Set." You can still buy one.
Y'know, it seems reasonable that there must have been more than ONE sheet of decals for the Space Ship Set, because on this sheet are no images to go onto the Klingon ship! Oh, well.
See you next time, campers!