Sorry for a late jump in Been packing bags... Uwe, I guess you don't have any pictures of the gun to which this mei is attached? Makes it so much easier to narrow down the smiths correct name. At least for me The big gun of the two that Mike showed a picture of got all the traits of a Sendai 10 monme matchlock. But going through the records of the gundmiths from this region (over 600) I can't find any that matches these kanjis. I came to about the same translation that you did, Uwe. There is a very similar looking gun to the above mentioned Sendai gun on page 61 in Sawada-san's book. But that one got three mekugi-pins. I hope Piers might be able to sving this one home, but I know he is somewhat busy right now.
You've narrowed it down to the correct Kanji though how the last name should be 'correctly' read can really only be answered by the smith himself. 信立 could be Nobutate, Shinritsu, Shindate etc,
There are two smiths of this line listed in my book, but their last names are Monsaburo 信竜 Nobutatsu/Shinryu and 信之 Nobuyuki. The latter made a 10 Monme gun listed in Kinokuniya 131. Both smiths worked for the Akita Han in 羽州.
Sorry about not seeing this thread until it was too late!