Yesterday I bought the original box for set 721, a 12 Volt locomotive. It came with the two named beams with 721 on it, a few other parts and a track contact base, and the latter is why I have some question marks. (I filled the box with my locomotive which I already had)
According to the inventory lists on Bricklink and Peeron this set contains not the track contact base however the extra inlay which you can see on the right hand side of the box reads in German: "[...] Please use the track contact base only with the transformer (12-Volt, direct current), article No 740 [...]."
Why would they mention the track contact base if it is not inside the set?
Notice the cap on the connector which seems to be only appears in the set 706.
Notice also the the spoked train wheel with traction teeth on the inside of the lid. I never saw this before and I even can not find it on Bricklink!
NB: the extra inlay was not found on the pictures I see when I searched the internet.