Isabella was born on 5-30-1854 per her census from 1900. The immigration information is a little stetchy as it looks like the year of immigration is before her birth (1851) but it shows her being here 43 years at the time so 1857 may be more like it. The "B" stands for Butcher.
Alvah, her husband, also went by Alfred E. Snow that year. If you still need more Snow genealogy let me know... I obtained a catalog from the family as I am a 2nd great grandson to him and the Snows were always such a mystery until I could access the info from the family directly.