ST743S is the tank number for either the round or elongated Drake with the GMax (1.6gpf) flush. The -01 is the color (Cotton White in that case). In other words, the round bowl GMax Drake and the elongated bowl GMax Drake use the same tank.
CST744S is the model number for the combination of tank plus bowl that makes up the elongated Gmax Drake.
That said, the flapper you want is the Korky "Fits Toto" Red flapper, model 2021PK at Lowe's. Here is a link to the page: Korky "Fits Toto" Red Flapper at Lowes The easiest way to make sure you get the right one is to pull it up online on their web site and buy it for pick up in store; that way, they find it and pick it off the shelf for you and have it waiting. Many local hardware stores, ACE, etc. have it as well. It's model 2021CM at Home Depot. It's model 2021BP most everywhere else. (In other words, you can usually ignore the letters after the 2021 as they change by retailer.)
Korky makes many of the actual fittings inside Toto toilets for Toto, and the flapper I mention is considered a factory-authorized replacement part, and probably is the highest-quality, best flapper you can get for a GMax toilet. Here's the page for it on the manufacturer's web site: Korky Red Flapper for Toto If you click on the "Instructions" link, it will show you a duplicate of the instructions that are on the box. In those instructions, Korky even suggests how many links of chain you should allow by model number of GMax toilet; that's how tight they are with Toto.