Edwin wrote on Nov 5^th, 2010 at 10:43am:

what's that extension attached under the tail part?

It was an addition to the the earlier707s for greater directional stability. I think it was because earlier versions of the 707 were harder to keep in a straight line, so the fins were provided to help the 707 fly straight. Later on, they were built with a taller vertical fin and so had no need the additional fin at the back.

From Wikipedia: These modifications also aided in the mitigation of dutch roll by providing more yaw stability.

Mark