India Post — No acknowledgement from addressee

Address:

Chennai, Tamil Nadu

I sent a registered letter with acknowledgement due, addressed to The Public Information Officer, Headquarters Deputy Tahsildar, Tiruvallur Road, Sriperumbudur on 3.3.2011 paying Rs.50.00 under reference number A 2950 from Vadapalani Post Office, Chennai-600026.
On 9.3.2011, I was delivered the acknowledgement card without any signature or seal of the addressee. This leads to the suspicion whether the letter was delivered or not. My letter being one under RTI Act, the acknowledgement is a must for appealing to the next higher authority.
I request you to take up the matter with the concerned and get me the proof of having delivered the letter to the addressee.