No, I haven't seen any records of sailings from Riga, either. But, you maybe should try the Hamburg Passports films from LDS. Evidently, ALL those leaving from Hamburg had to have a passport to board ship and the dates are in your range and they list date and place of birth for the emigrants. And there is an index!!!!! Maybe this will lead you to their origins in Germany (assuming they took the same route as mine did!).
I have the same situation with some records saying Germany, others Russia for the same person!! Good luck with the church records.