There was another Baden thread here a while back that showed the secret mark on one of the other values in this series. (The secret marks move around just a bit.) For the archives, the secret mark on this 3kr value, above, is here:

It is the little protuberance on the otherwise uniform scalloping.
The small "5" in 1850 and the capital "F" that doesn't touch the frame are still spotting features of genuine stamps that do carry over across more than one value.
The presence of all 3 is still no guarantee of genuineness, but the absence of any one or more should confirm a fake.
(I'm far from an expert on this German stuff, so if anyone sees a flaw in this, please, point it out.)
As for the post office numbers, I don't have a rarity guide, so I would base my personal guesses on relative populations of the towns (making my guesses completely unreliable for valuation purposes).