iPhone SHSH for go back to 3.1.3 I have a iPhone 3GS with iOS 3.1.3 in my job. I have to update it to 4.1 and after I need to go back to 3.1.3. We have only this iPhone with it, so if I do something wrong, I will never can go back.

I run TinyUmbrella and it save the SHSH for 4.1 , 4.3.5 and 5.0bx iOS.  But, why it don't save for 3.1.3? This mean, if I update it, I will not go back to 3.1.3??

Why it not save for other 4.x versions?? Am I doing something wrong?
TinyUmbrella can only save the SHSHs for firmware versions that still get signed by Apple. 3.1.3 isn't signed by Apple anymore (i.e. they don't want you to downgrade to it), and therefore you don't get the SHSHs anymore.