Not that I'm an expert on woman's rights organizations, but I had not come across this group before. They aren't in Rochester History, nor are they in Nancy Hewitt's Women's Activism and Social Change: Rochester, New York, 1822-1872, which is probably the best and most comprehensive book on the subject, or her published work on Amy Post. Obviously, this Union was short-lived and did not generate many records.
According to Lisa Tetrault (among others), woman's rights activists resisted forming organization -- or at least national organizations -- in the antebellum era, working through temperance or antislavery societies. Also, in 1848, a woman's rights movement as such really did not exist. There was a sense of woman's rights, and women advocated for their own rights, but this took place within or in tandem with those other types of organizations. Even the Seneca Falls meeting was considered a blip rather than the beginning of something momentous. Although they knew of that convention, the woman's rights activists tended to date the beginning of the national movement to the 1850 Worcester Convention.
So, this Woman's Protection Union is an anomaly. The members formed it on 18 Aug 1848, almost exactly a month after the Seneca Falls Woman's Rights Convention and two weeks after the more publicized (at the time) Rochester Woman's Rights Convention. Of its founders, Amy Post attended both conventions, signing the Seneca Falls "Declaration of Sentiments," and Sarah Owens and "Mrs. Roberts" spoke at the Rochester meeting. (I still have to do some investigation into "Mrs. Cavan") Both Owens and Roberts were concerned with discrimination against working women, which was clearly an important issue with the Woman's Protection Union given that they planned to continue their schedule of lower dues for women until "the time shall arrive that woman receive and equality of remuneration for the same labor performed as men." The meeting place, too, seems to have been a union hall, with the full name, "Hall of the Mechanics' Protection."
This is also an attempt to form a reform society focused specifically on gender discrimination, placing it among the first -- if not the actual first -- explicitly woman's rights organizations in America. But, since I am not an expert on early organization, perhaps this last point can be debated.
Of course, Frederick Douglass was connected to it.