Neither did the possession not do the sale deed/registration is key to the acquisition of property.
According to Bangalore bench (2021) 23 ITR (trib)-OL 625 the acquisition is taken as the date by when substantial payments ( in this case nearly 70-75%) are made . In this case the agreement and allotment got made in 1996 when substantial payments were made and it is only that the balance of 25-30% were made in 2006 while taking possession and execution of sale deed. The assessee sold the property a year after in 2007 to which the AO alleged as short term sale transaction.
This assessee got indexation from 1996 onwards as also an acknowledgment to the transfer of long term capital asset.