Mercadier claims it is the only one of its kind, made at the Leblanc factory in 1971, although according to Wikipedia they made three contralto clarinets and one octocontrabass clarinet. This coincides with the Octocontrabass page at Grant D. Green's fascinating site Contrabass.com, a collection of information on the lowest of the low wind instruments.
|Mr. Leblanc himself playing the octocontralto clarinet (photo from Contrabass.com and courtesy of G. Leblanc)|
And for a truly odd musical coupling, Mercatier and Benjamin Masciotta perform Tchaikovsky's "Dance of the Sugar Plum Fairy" on the octracontralto and the A-flat piccolo clarinet!