I see and read some of your other postings on John Leitz (Leetz) on the Fond Du Lac board, and am not related. Just trying to help and caught my eye since my KRUSE family came from Germany in 1869, some went to Chicago and some to Wisconsin in Shawano County. Many German immigrants settled in the Great Lakes Region.
Anyway, the census immigration dates you had showed anywhere from 1865 to 1870, and it looks like John came alone, as his wife was a different date.
I did a look up in my Germans To America (GTA) CD and see a John Leitz with the right age. He came by himself.
John Leitz, age 26, arrived 10 July 1865, origan of Germany,
to U.S., port of embarkation, Hamburg and Southhampton,
ships name: Saxonia, occupation merchant, ship captains
name: E. Meier. The GTA listing is the "arrival passenger list" at New York port.
If this is the right guy, then with this info you could order the Hamburg Passenger ship departure listing from you FHC and this should show the town in Germany. Then from the town you could order the church records that serve the town. There is no "birth certificates" for that time period in Germany, just the church records of births and christenings.
Let me know if you need any help ordering this microfilm. I have done this many times with great success. This is exactly how I found my Kruse family.
A question, why do you think he came from the Posen area? Have you any luck with his naturalization records? Apparently the application for naturalization (intent) should have info on origan and immigration.
My Website: Kruse Family from Germanyhttp://freepages.genealogy.rootsweb.com/~fkruse/