Hi matteo! Ok, I must have misunderstood you about having an original ST/SR
Do you mean "there were (no) M491-specific shells built at any time as far as I am aware"?
But, just looking on my own car (believed to be Waldegård or Åke Anderssons ex rallycross -72 car) there are details on the shell that can´t be modified after the body was assembled as
some parts (like the dead pedal ect) is spot welded on before the inner box section. I´ve seen on later cars as RSR -74 etc that the dead pedal is not spot welded but welded unto the body with
seam welds. There are more details, like the 2 clips that holds the oil breather bottle in the engine compartment. They are also spot welded, and doing that after the body is finished off the line makes little or no sense.