For Squirrel41 and Torque...
The AMBER loss is still open to question but, according to one of the crew who was not on the flight (if I remember the last presentation I saw), discussed the lack of completion of a TCTO which might have caused a problem. Not related to anything about current RIVET JOINTS or the -135 airframe in general.
With regards to the 707 tail sections being used on the -135: When the decision to re-engine some KC-135As with TF33-PW-102 turbo-fan engines from retired 707 airliners the increased thrust also required the use of the larger, broader span, horizontal stabilizers and elevators. ALL RC-135s with turbofan engines (original build and mods) had the larger horizontals as original build. A much earlier ('50s) Boeing modification increased the original -135 "short tail" to the standard 707 height.