Yes, the line is on the outer pair of contacts.
If you place a BT431 plug with the contacts uppermost and the latch on the right, then the A-wire (tip) is on the left and the B-wire (ring) on the right. These correspond to terminals 5 and 2 respectively on a BT socket. Pin 3 is used for ringing and is connected to pin 2 via a 1.8uF capacitor in the master jack, so if you're using a 4-wire cord make sure the other end doesn't connect to anything which will be affected by this.
By the way, thanks to a major lack of communication, when these plugs were introduced in 1981 we ended up with the official standard numbering the pins on the plug the opposite way around to the terminals on the socket!