Not so random, since it follows a nice, predictable curve. Back when I first bought the game, which I think was shortly after Infocom went out of business, it turned out that someone had removed the forms from the box. I suspect it was some employee at the store who “borrowed” them, because someone had also stamped the name of the store on a couple of the remaining feelies.
It was their last copy, and I seem to recall that they offered to take it back because they had no idea where those missing feelies were. I decided to keep it. The picture on the back of the box gave me an idea that there was a 3-digit code that depended on the current time in the game. While I never figured out the exact formula, once I had enough data points I could make pretty good guesses for the ones I hadn’t yet tried.
Now that the Infocom source code has been leaked to the Internet, we can of course see exactly how the code was written.
I assume you mean Zork 3?