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The parts catalogues of 906 & 907ie say 73740021A. This number was replaced by 73710011A which is also the same as 73740031A and 412806091F, 412806091F.

These belts are listed for
Ducati 900 SS -`92
Ducati 900 SS/SL`94-`97
DUC 900 Final Edition , BJ 2002 - 2002
DUC 900 Sport , BJ 2002 - 2002
DUC 906 Paso , BJ 1989 - 1990
DUC 907 Paso i.e. , BJ 1991 - 1992
DUC Monster 900 , BJ 1993 -
DUC Monster 900 i.e. , BJ 1998 -
DUC Monster 900 i.e. Dark , BJ 1999 - 2002
DUC Monster 900 S , BJ 1998 - 2001
DUC Monster 1000 Dark , BJ 2003 -
DUC Monster 1000 S , BJ 2003 -
DUC SS 900 i.e. Nuda , BJ 1998 - 2001
DUC SS 900 Nuda , BJ 1991 - 1991
DUC SS 900 Superlight , BJ 1992 -
DUC SS 1000 DS , BJ 2003 -
DUC ST 2 , BJ 1997 -

73710081A (as successor of 73740023A) is listed for
M900-ST2 01

(source Stein-Dinse web shop)

Summarized they`re all the same (70 teeth). What makes it confusing is that Ducati often changes part numbers i.e. when they have different suppliers, new models etc.
The belts of the Cagiva Elefant 900 and Gran Canyon 900ie are the same, too.

G.

Hi Ducati Pete. I think paso750 has answered the question about the belt part numbers very well.

It is fairly easy to realign the pulleys afterwards just get all the timing dots in the right place, before you push the belts on. There was a tendency for the pulley to move slightly so just try to keep everything aligned without moving and check it out afterwards. If I can do it !