Mozart K310 - what's that X in the autograph manuscript?

[charles65]

In bar 29 of the first movt, there's a X indicated above the left hand in Mozart's hand. I haven't seen this before. Does anyone know what it indicates?

[kosulin]

A tie from d in bar 28 to d in bar 29.

[brogers70]

I have no idea what that X means. If you look at the first edition and then the Artaria edition, both late 18th century, you find no ties on those D's or the similar situations in the following bars (first edition) or no ties at all in that passage except for the tied D's. I personally think that it sounds better with the ties, including the D-D tie with the X over it, and I think I remember always hearing it played that way. But basically I have no idea if the X means anything at all.