According to
this page, Bio-Tech has two official abbreviations, BIO and BT; however, BT is also Blood Types (a book for Gurps 3rd ed.). So my guess is Bio-Tech used BIO for 1st edition (for GURPS 3rd ed.) , and BT for 2nd edition (for GURPS 4th ed.).
Would someone connected to SJGames either confirm or deny this?