stilton
...there is what appears to be a fixed red line marked on the airspeed indicator opposite or pointing to around 190Kias...
Could you please show us which picture you are looking at, as there was no such red line on the ASI on BA aircraft and I can't think what purpose such a line might serve?
The only thing I can think of is that there was a VMO Failure Flag window, cut into the face of the ASI dial, adjacent to 190kts.
When the flag was displayed, this indicated a failure of the VMO pointer, something that was very important on Concorde.
This flag had an all red background and bore the letters VMO on it in black.
I just wonder if perhaps what you are looking at might be the edge of the VMO Pointer Failure Flag which has not fully retracted into its window, leaving just its lower red edge displayed, which could look like a red line.