As per the details provided by you, it appears that you do not any documentary evidence before your Date of Registration in March 2013 other than your Bank Statement.
In such a case your date of purchase shall be March 2013 and indexation shall be considered from March'2013 unless you want to fight it out and claim that the Date of Purchase was August 2011 which again involves time lag, legal cost and professional fees. So consider all the details and plan your decision accordingly.
Trust this clarifies your query.
Feel free to call / get back in case of further clarifications.
Rohit R Sharma
BCOM, ACA, LLB-GEN, CERT. FAFP.