Regarding C-135's , there have been a couple based at Mildenhall recently with 57- and 58- serials that must be some of the oldest around. I'm surprised that there are still 707/C-135 variants in the US forces still flying with the old engines, such as the E-8 mentioned earlier. I suppose that they have enough spares for a few years more and the old adage "If it ain't broke, don't fix it" probably holds true!