The following is excerpted from a September 22, 2010 post by Christopher Jagalla at Intellogist:
Acquiring correct assignment data can necessitate more effort than just looking at the assignment field in your favorite patent search system or on the face of the patent in question.
There are several avenues of inquiry suggested to find out what your assignment searches might be missing, so let’s talk about one specific way to improve them today–searching USPTO Assignments on the Web.
USPTO Assignments on the Web is a free patent assignment information tool through the United States Patent and Trademark Office’s website. This tool has assignment data from August 1980 through the present, and users can query several fields of information.
USPTO Assignments on the Web displays hundreds of results for assignee/assignor searches easily, but that’s not to say it’s the ultimate solution. It’s always best to consult multiple sources of assignment data, including standardized assignee data that can catch spelling variations or even automatically incorporate corporate hierarchies.
The “conveyance text” portion of a record can be of special importance: This section indicates the type of reassignment, which can include an “Assignment of Assignor’s Interest,” “Name Change,” or “Security Agreement.”
See the full post here.