Another good mention is the final hidden P-63C-5 variant of the Kingcobra in the Soviet tech tree first available for a Winter event back in 2015 along with the VK.4501 (P) Tiger.
Which honestly shocked me since I only ever thought they had the P-63A-5/10 as premiums only.
You edited your reply with the image after I replied to you.
And yes, I see it now. I wasn’t aware that an event vehicle would be folded under the tech tree IS-2 1944. That’s a random place to put it, and it doesn’t make sense.