Big difference from the Lady's version in the Zenith forum - look at the relative size of the date window.
Yes, this one is a 1000m behemoth. The 250m lady's version is for dainty ladies who only dive down to 250m.
It's just an extension of the tachymeter onto the second rotation of the chronograph second hand. There must have been something significant about 50, because that's a feature of all the later Eberhard chronographs. Perhaps 50kph was a speed limit?