When Luther's house in Wittenberg was excavated about 20 years ago, a golden ring[2] was found that must have been deposited there before 1540. It is therefore quite likely that this is the ring mentioned by Luther in 1537.
[1] See WA, BR 8: no 3162 -- https://archive.org/details/werkebriefwechse08luthuoft/page/...
[2] Here is an image of the ring: https://www.zum.de/Faecher/G/BW/Landeskunde/rhein/geschichte...
My Catherine thanks you for the golden ring, whom I have hardly ever seen more indignant than when she realized that it had been stolen or lost through her own negligence (which is not likely for me, although I still insist on it), which I had persuaded her that this gift was a happy omen and augury sent to her, as if it were now certain that your Church would agree pleasantly with ours; this grieves the woman wonderfully.
edit: Not code, just convention.
The more practical reason to mount ground down is that wall warts with ground pins or polarized prongs nearly universally arrange them so that they're hanging down when inserted into a ground-down plug. If the plug's flipped, the wall wart's upside down and its weight is trying to lever it out of the wall.
TBH, in the house I mount them ground down, but under cabinets or in the garage/shop or etc I mount it ground up.