SF7 is not a serial number.

http://www.savageshooters.com/attach...8&d=1327338525

Small shank barrel is on the left. Notice the step in the shank of the large shank barrel on the right.

http://www.savageshooters.com/attach...9&d=1327338746

Small shank again on the left, notice the difference in length of the barrel nut.