One last admission from my side to end this energy discussion:
When I simply enter the 275 kts into the simplified formula of an E6B calculator I get 467,5 kts as TAS.
When calculating considering the Mach values I get between 475 kts (M0,80) and 490kts (M0,82) for an ISA temperature of -54,3° (ISA standard atmosphere at 35000 feet).
Depending on the real temperature this value can vary significantly and therefore the TAS can probably have been easily been 460kts as well as 500kts. We shouldn't read too much into it without knowing OAT.
Mach dependent TAS:
a0*M*sqrt(T/T0)
where
a0: speed of sound at S/L
M: actual Mach number
T0: ISA temperature at sea level in Kelvin
T: actual temperature.