For the sharp-eyed who may have actually gone back to look at Bellerophon's picture, you may note that the AFT bug is lower than M1.56, contrary to the flt envelope above. Billy ruffian will know for sure, but here's my surmise:
FL600 level flt means he was going to BGI. The length of this sector was, in crude terms, about 200NM more than the quoted max range of the aircraft, so the range envelope was being pushed a little.
Because there was no land you could stay supersonic all the way, so at the end of the cruise you would be supersonic, but with relatively little fuel in the tanks, and most of it in Tank 11 (at the back) to keep the CG aft. Even with a tweak to tanks 1&4 to run them at 50% level, eventually the CG would come forward as you burnt fuel out of tank 11. That's what is probably happening in Bellerophon's photo, hence the 'AFT' Mach bug being at a lower Mach. If the FCPI ('ICOVOL') was in the frame I wager you would see the elevons a somewhat above the optimum 1/2degree down position
The bugger was this; if you were a little tight on fuel, just when you wanted to maximise the time spent supersonic you'd have to start an early decel because there just wasn't enough fuel left to maintain the CG far enough aft to sustain M2.
All part of the fun, and why every sector was interesting and rewarding.