[To summarize the lengthy discussion below, the grey flapper is the new PVC version of the pink-with-float Chlorazone flapper that used to ship with your toilet. If it were I, when it comes time to replace it, I would just get the Korky 3" large adjustable flapper, Model 3060 (and whatever lettered suffix), set it and drop it in. It will work perfectly.]
The Korky red "Fits Toto" flapper (Model 2021) is the right one for the 1.6 gallon Gmax toilets. The Blue "Fits Toto" flapper was also meant to flush 1.6, but it is designed to flush 1.6 with the old Power Gravity flush, which doesn't fill the tank as high as the GMax flush does, so the blue one in effect drains the tank if you drop it on a GMax or EMax flush toilet, making it closer to a 3-gallon-per-flush monster. The extra water doesn't add much to the flushing ability of the thing, so the blue one is obsolete for newer toilets, but probably sells OK still because it's right about the time where a lot of those older Power Gravity toilets are going to be needing a new flapper.
The Red "Fits Toto" flapper (Model 2021) is the equivalent of the Toto THU175S flapper. (Actually, it's the exact same flapper as the one made by Korky for Toto and just put in different packaging.) So any toilet that takes the THU175S will be very content with that flapper -- and a lot of Totos do. Toto has now come out with a new flapper made from PVC instead of the Korky Chlorazone material used in the THU175S (and Korky 2021). That is the THU331S, and it's pink PVC. My two Original Drakes, and my Carlyle II, all are designed to take the THU175S, so I will be replacing their flappers, when necessary, with the Korky 2021; although the THU331S is the latest, I'm quite content with the Korky 2021 flapper (which is widely-available and half-the-price), and so will continue to use it.
Your Drake II used to ship with the THU253S (pink with float) flapper, for which the equivalent PVC flapper is the THU347S -- the grey PVC flapper -- which is now what is being shipped from the factory in the toilet. There is no direct equivalent Korky part. However, Korky makes a "universal"-ish 3" Adjustable Flapper, where you just slide a little plastic sleeve around the outside to increase or decrease the buoyancy of the flapper and thus the amount of water that leaves the bowl. Many smart plumbers just carry that part on their truck, so that one flapper will work perfectly on basically every Toto gravity toilet with a 3" flappered flush valve. Terry is one of them, if I read one of his earlier posts correctly: one Korky flapper and one Korky fill valve basically let him fix almost every Toto. That flapper is Korky 3060BP (you can ignore the letters -- its 3060 and some suffix at almost any hardware store, the suffix denotes the packaging format: blister pack, etc.).
http://korky.com/Flapper3060BP.html It's what I would drop on a Drake II if I owned one. [For purposes of precision, certain toilets, like the floor-mounted Aquia dual-flush, Maris double-cyclone dual-flush, and "1G" toilets do not use a flapper, and thus can't be fixed with the 3060BP.]
There is a nice little illustrated chart from Western Sales that lays all this out. Of course, being a Toto piece, it has typos. I don't know why Toto's printed materials always have typos, but they do. In your case, your tank is ST454E. The Toto piece has no reference to that tank number, but it does refer to "ST545E", which doesn't exist. (One other clue is that they refer in that piece to ST545E(R), meaning that it applies also to the right-hand trip lever version of that tank, and only a few Totos can be special-ordered with a right-hand trip lever and the Drake II is one of them, so clearly they just typed 545 when they meant 454...) Soooo...if you peruse the flapper chart that I give the link to below, the tank for your toilet is incorrectly listed as ST545 instead of ST454. Here's the link -- it's actually kind of interesting (and it has PHOTOS!!!), but maybe just to a limited audience.
http://western-sales.com/mpu/jun12mpu/TOTO Flapper Conversion Table.pdf