As Grunherz said. tailwheel was retractable from the 109F models, and it remained so until the G-6 (late production G-2s and G-4s introduced a larger tailwheel that did not fit in, same as fitted to G-6. So some G-2/G-4s did have non-retracting ones) Later Gs introduced longer tailhwheels to raise the tail on the ground. The 109K used the same long tailwheel, but then it was retractable again, with cover fairings.
BTW, afaik the self sealing tanks were already there on the Emil.