My best guess is this is an early production square based on the Athol stamp, the numbering and the fine stipple finishing on the head. Excellent condition with intact bubble and noting it is missing it's scribe.
From industrialhistory.org :
"Athol Machine Company is the grand-daddy of tool manufacturing in Athol. This is the company that was formed in the spring of 1868 to produce L S Starrett's invention of the meat chopper, and other inventions that may arise in the future. Same company that started making vises c1873.
Starrett got in a dispute with the other owners, left the company c1877, and following his invention of the combination square right around that time he went independent, establishing what is today's L S Starrett Company in 1880. Then as he brought out other tools, especially measuring tools, calipers, etc., Athol Machine started copying some as well as developing their own unique items. Starrett sued, eventually won, and 25 years after he started his own business he bought out what remained of Athol Machine, put money into it, expanded the vise production by a great amount in a new factory he built for them on that same spot, and in 1921 it became known as Athol Machine and Foundry. Starrett still controlled the business."
