I'm a bit stumped, and there is probably an obvious answer that I'm not seeing, but here goes. I found my ancestor in Ancestry's "Essex, Massachusetts Probate Records, 1638-1840" database:
File #: 29672
Name: Ebenezer Whittemore; Whitmore; Whittimore; Whittmore
File Date: 10 Sep 1839
Comment: case beyond 1840
Now, this database is apparently an index to the original probate records, and apparently, I'm meant to get to these original records and use the file number above (29672) to locate my ancestor's actual probate records. However, this database's source information doesn't give any clues to how to go about this, and the original probate records don't seem to be available here on Ancestry either.
Most of my research has been here on Ancestry using available sources, so I'm not sure where to turn. I'd appreciate if anyone could give me a push in the right direction. Thanks!