Yes, 73 notes indeed.
The original instrument was built by Cavaillé-Coll for Alexandre Guilmant (Dupré's organ teacher), who also lived in a villa in Meudon (just a stone's throw away from Dupré's). It had only three manuals, but already a 61 note manual compass, which was quite exceptional for Cavaillé-Coll. Guilmant died in 1911, but his organ remained in his villa for some time after that.
When Dupré settled in Meudon in 1925, he heard that Guilmant's heirs wanted to get rid of the organ, so he bought it. Then, in the 1930s, he transformed it quite extensively, adding a fourth manual, several stops, a combination system (looks funny now, but back then it was state-of-the-art) and that famous sixth octave to the manual compass.