This is a single Shimano thumbshifter for the front derailleur. If you are asking where the other thumbshifter is I am sorry to say I do not know.
I bought this one because I had a right thumbshifter, but when I received it I noticed that they where not of the same model number (how stupid to miss that).
Anyway, there are still loads of uses for a single thumbshifter.
This SL-M730 left Deore XT thumbshifter could be run anywhere you want to shift your front derailleur on your MTB.
I am unsure if it would work on modern drive trains but a wild guess is, why would it not?
You could also use it to replace some parts on your current thumbshifter if you are looking for spare parts.
Anyway, I can find lots of uses for it and I bet you can to, so that is why I am releasing this fine Shimano Deore XT shifter for 7 euro. Sold back in october 09.