This guilotine blade is the one that beheaded Marie Antoinette in 1793. 
It was purchased from the executioner’s family.

It can be seen on display at Madame Tussauds was museum in London. 
Marie Antoinette’s last words as she climbed onto the guillotine platform were “Pardon me sir, I meant not to do it” after accidentally stepping on her executioner’s(Henri Sanson) foot.