The technique for foil stamping during the printing process uses an aluminum "die". This die is a heavy slab of metal with the image to be metal-stamped embossed, or raised, on one surface. If you look at the kit's Parts Sheet A, you'll see that there is a LOT of silver foil. That means the die was pretty big, about 8.5 by 5 inches. Everywhere you see silver foil on the paper, there was an embossed area on the die, which was made by a photo-etching process.
When the die is used in the press to fuse the foil to the paper, it is electrically heated. OK, you probably get the picture now. The die EXPANDS when it is heated! And its temperature changes during the stamping process.
Our expert printing people knew this, but they had never used such a big die, with so much image area while at the same time working to a tolerance to fit the laser-cutting. We all misjudged the expansion factor. Of course knowing this factor exactly now, the next SCI KITs to be created would all take advantage of the experience.
We didn't feel that it was worth making a new die, and risking different problems, since all the other silver foil areas on Sheet A worked very well, especially since they are all trimmed by the laser-cutting process. And the Primary Mirror is completely covered over by other parts in the HST SCI KIT, anyway!