After having stood in the Place de la Révolution during the deaths of Louis and Antoinette, the position of the guillotine was moved around. I don't know if it was the same one that was transported back to that square for Robespierre's execution, but it's possible.

After Robespierre's death, it was moved around a bit more, and eventually ended up at Place de Grève, where some famous-but-not-THAT-famous people like Jean-Baptiste Carrier and Fouquier-Tinville were executed.

The reason I bring that up is that there are a few blades connected to the executions on Place de Grève. One is in the museum of the Paris Prefecture of Police (link with tiny picture), and the sign says that it was used in that place during the revolution. However, the guillotine was also placed there originally, in 1792, before the Revolutionary Tribunal decided to have it moved in 1793 to Place de la Révolution.

There's also the blade used to execute Carrier, which would have been in November, following Robespierre's death in July. It's been preserved, and here's a picture with some information. However, the fact that no one seems to mention Robespierre in connection with this blade makes me doubt that it's the same one. It's my best bet, though.

There's a blade on display in London claimed to be the one that killed Marie Antoinette, and purchased by the executioner's family. However, the fact that it was acquired for the Madame Tussaud collection makes me doubt it. The lady herself was a lying liar who lied, especially to create connections between herself and famous individuals. Considering that the exhibition is based on her work, I wouldn't expect her heirs to be much more attached to historical fact than she was.