Section 24 of Income tax - Act

The very purpose of possession letter is to prove that the house is actully 'READY FOR USE". Since Completion from local authority is a procedural part & takes lot of time, the best evidence that the house is ready is the possession letter. In your case, the sense of 'possession given on completion' is missing. So it would be difficult to justify on the face of the letter that the house is ready for use. Hence the interest could be claimed u/s 24 in the financial year in which the possession to stay is given by the developer.  

Since you have mentioned that it is no were written in the letter that construction is complete, the same cannot be used for Section 24.