The same rules as 2nd ed (which were separate books) but in one book.
I'd like to buy that instead of the separate books (unless my imagination is playing tricks on me )
Ok, lets see ( It's going to be easy, I have everything SM/SM2 not far ).
SM 1st Edition contained Tech Law and Future Law ( in a boxed set, but you could have them as stand alone books too )
Spacemaster Companion ( unnumbered ) was for that edition.
Then came SM2, the content of the above companion was included in the rules, and all the vehicular design and combat from the Tech Law and the SMCu was removed and put in other products ( Namely : Star Strike for Space Combat and Armored Assault for Air/Land/Sea combat ).
That Spacemaster 2nd Edition originally came in a boxed set that contained the Player Book, the GM book and the Tech Book.
To this was added the Spacemaster Companion I.
Then a bit later was released a Second Printing ( well what I call a second printing ) of SM2 as a single book ( #9050 ).
This book only contains the Tech Book, the GM Book and the Player Book under a single cover.