Not available for gold (it’s not a calendar item) but for sale in the store for $14 USD, $45 USD, and 67 USD, depending on the package.
Introduced in 2014, this was originally only available through the gift shop (is that still a thing?) and not on the tech tree.
It was in the game at launch and was supposed to be released in box sets so until version 9.0 it was never seen in the game.
It’s a captured German tank based on the Panzer III hull with an 85mm gun on a fixed turret.
At one point, it was once one of the rarest tanks in the game.
A 2014 review of this tank suggests it wasn’t worth the 10 Euro price (in 2014 money). Here’s a link to the review. Apparently, this tank is often the victim of ram kills and single shots from virtually every tier 6/7 tank.