Thank you for such a detailed and helpful response. As I have writtten to Ines, I also had wondered if the two dates mught be the date of the entry and the date on which the event occured. Three of us have had the same idea, so I am beginning to feel that this is the answer.
Your suggestion that he went to WIesbaden and Landau for education is certainly a possibility. But he was a butcher (as was his father and grandfather) and the family was poor. So education seems less likely. I wonder if he might have gone to one or both those places for work. But, as far as Wiesbaden is concerned, his stay there begins in October 1913 and continues until November 1916, mid-way through the war. Maybe he signed on for extra military service and was then attached to Fusilier Reiment 80, which was based in Wiesbaden. But then the card entry would not be 'o/P'. I find that all difficult to explain!
Fantastic that you have a possible deciphering of 'o/P'! I have struggled with this for many months and your suggestion would fit well. I will investigate further when I am next in Offenbach and try to see if it can be confirmed by looking on other cards.
I was aware from the Verlustlisten that Friedrich was wounded in October 1914. I also have information from the Lazarettbuch in the Bundesarchiv that he was discharged five weeks later, declared fit for duty and was sent to a replacement battallion as a Reservist ('Abgang am: 11.11.1914 nach: dienstfähig zum Ersatz-Bataillon'). The next information i have comes from 11 letters that he wrote between April 1915 and January 1916. By then he was then in Infanterie Regiment 87, 6 Kompanie and serving in France. But after January 1916 I have nothing, except that the Meldekartei shows his return from Wiesbaden in November 1916. He may have been wounded again, but his name does not appear again in the Verlustlisten, So what happened to him then is still a big unknown.
Thank you once more for your great suggestions. I am very grateful and would welcome any further ideas you may have.