It appears that was just the chapter name of the Bethel, Maine chapter of the Order of the Eastern Star (as pointed out by Steve Bird in the question comments). This appeared to have been originally intended to give female relatives of Masons some way to belong to the organization.
"Purity" is just a codename for that chapter (or possibly for that branch of chapters), probably intended to be easier to remember than a number when one is dealing with multiple chapters.
For example, the first chapter appears to have been named "Alpha Chapter no. 1". 1 being its chapter number, and "Alpha" its chapter name.
The WP page on the OES states one chapter being established was known as the "Queen Esther Chapter No. 1". That chapter was quite possibly the first predominantly African-American chapter, as it appears to have been associated with the predominantly African-American branch of the Masons. So it looks like the numbers themselves may only have been unique within the Masonic branch that the OES chapter was affiliated with.
This appears to be a very female-centered sub-organization of the Masons (although men were allowed membership as well). Their public iconography is a 5-pointed star, each point of which is colored differently to represent a different Biblical woman and a virtue they associated with her.