Since Ancestry adds databases continuously, many overlap. Once they're added, Ancestry doesn't seem to coordinate them with the others. :(
Another resource that you might not know about is fultonhistory.com. I did a Boolean search there for Esterbrook w/8 pen and got over 2700 hits! (Most are ads, though, I'm supposing.)
Do you have access to GenealogyBank? A search there for "Esterbrook Pen" brought up over 700 hits, including trademark cases, Mrs. Mary Esterbrook's obituary from1899, and an article re: Esterbrook's exhibit in the International Exhibition of 1876.