To OP GM Joe;
I don't suppose you can remember the year you bought the set with this booklet in it can you?
I have a suspicion that it was probably late in the life of CT and it was produced to use up stocks of deluxe boxes - I know they made a deluxe set with the softback Traveller Book in it.
You may be right. The original Deluxe Starter Traveller set I bought back in the early 80s didn't have this booklet in it -- or if it did, I both lost it and forgot about it.
After I graduated from college and moved to IL, I discovered Games Plus in Mount Prospect. They had quite an extensive selection of Classic Traveller merchandise, all marked down! So I bought a bunch of stuff, including Starter Traveller and another Deluxe Starter Traveller. Of the two, only Deluxe Starter Traveller had the Additional Rules booklet in it.
I remember opening them both and looking through them (these were backup copies, and haven't been used to this day) and I saw that Additional Rules booklet and thought, "Crap, I must have lost my original one. It would have been nice to have had these rules back before I bought The Traveller Book and discovered the experience rules..."

That trip to Mount Prospect was probably sometime in 1993 or 1994. I remember being not-at-all confident in my ability to drive there and back without getting lost, so it was not too long after the move.
This all came up because I'm gearing up for a Traveller campaign, and was going through my CT stuff and decided to open the still-new Deluxe Starter Traveller box and take a peek at a fresh copy of the game that started it all for me. That's when I came across the Additional Rules booklet and remembered wondering whether I'd lost my original one, or if it had never had it. So I looked at the CT CD-ROM contents, and there was no sign of it. That's when I decided to ask on here, figuring someone else might know the answer as to the origins of this book, or if not, maybe we could figure it out together.
As it turns out, it appears to be rather rarer than I thought. I'd be happy if this ends up adding to the preserved materials available to the community.