The initial B707  Rudder-design was  for a manual Tab, a "balance" Tab:  no hydraulic-boost.  

The Rudder Pedals were connected to the Tab,  the Tab then flew the Rudder.  

Initial Fin-Rudder  design:   a shorter-height VertStab, Rudder --  manual, spring- Tab   [no hydraulic boost].

The early B-707 Rudder was operated only by aerodynamic forces on the TAB  and by differential pressures acting on the BALANCE PANELS  ahead of the rudder Hinge Line on the fin. 

Per _Flight_,   25July58,   p145:  

"Flying Controls  Internal balance platesaerodynamic tabs . . . are the basis of the Boeing 707's mainly manual control system, which also provides proportional feel,   but rudder boost is used on the 707-220  and Intercontinental models  for maximum rudder deflection in the low-speed, engine-out case...."

 . . .  the beginning of rudder "blowback,"   where the rudder hinge moment  is overcome by  slipstream forces  and  causes  the rudder to streamline ​ . . . 

Modification center,   at the old B-17 factory at Plant-2,   included  the  Fin-Tip Extension.

The various Fin-Rudder modifications continued through the next decade,  with the Rudder-TAB  rotation  redesigned to  an  Anti-Balance function, with various "boost"  iterations  [from the initial manual-Tab  to a "power augmented rudder"  then later a "full range boosted Rudder"] ,  and  various iterations of  Rudder-Tab ratios.  The artificial stability was added:  first effort was the  "parallel" Yaw Damper,  then after AA Flt-One,  a redesigned  "series"  Yaw Damper.

But what was the real cause of all the loss of control training accidents?      

Investigators of the 1960's, '70's, early '80's,  would have you believe that it was simply a case of pilot error.

As we now know it [training mishap fatalities] were a result of poorly designed and improperly certificated airplanes ... attempting to fly with thrust-asymmetry [engine failure] at too low speeds.   

This hindsight conclusion is not meant to cast any aspersions on anyone . . . but only to put into  perspective the evolution of the design of jet transports. . . . To understand this, it is necessary to review  just how the aircraft were certificated for  minimum control speed.

Marthinsen, H.F.  "Engine-Out Flight Training Revisited". Presented at ISASI's  13th Int'l Seminar, Tel Aviv, Oct' 11-15, 1982; ISASI Proceedings 1982,  p46-63. 
Probable Cause The Board determines the probable cause of this accident was the crew's failure to recognize and correct the development of excessive yaw which caused an unintentional rolling maneuver at an altitude too low to permit complete recovery.

[C.A.B.'s  AAR,   p11.]      

By February 1960, more than a year before the C.A.B.'s accident report was adopted,  the manufacturer had initiated   Fin-Rudder modifications:


Fin-Tip Extension,  and the hydraulic-boost, with an anti-balance Tab.

Failure - Interactions

Details of the C.A.B.'s   "Peconic"  investigation . . .   [in-progress,  standby ... ] 

A public hearing was ordered by the Board and held in two phases.   The operational phase of the investigation was held at the Henry Perkins Hotel, Riverhead, Long Island, New York, on  August 27, 1959.   The technical phase of the investigation was held at the Forest Hills Inn, Forest Hills, Long Island, New York, October 7, 1959.

Press story reported on first day of Public Hearing, Thursday, August 27th 1959:

Thrust asymmetry, yaw, roll.