Apartment sale - LTCG or STCG? One time payment

Hello Experts, Kindly clarify below scenario. Thanks in advance 🙏

An NRI purchased a residential apartment in full payment mode when the project was launched in March 2021. Builder provided an allotment letter (dated as of the payment completion date) mentioning the unit number and the payment details to the buyer. 

The project is close to completion now after 3.5 years time. If I decide to sell the apartment before registration, will the gains be considered as long-term capital gains (LTCG) or short-term capital gains(STCG) ? 
Assume profit could be around 1.2cr roughly.

I’m hearing that LTCG holding period starts only after registration is complete. When I googled about this, it's also mentioned that date of acquisition is when the clock starts. Can the date on the allotment letter be the date of acquisition?  Attached is what ChatGpt says. Can we claim as LTCG as we got allotment letter 3.5 years ago itself?