Architecture Puzzler #138

Insulating glass (using two panes of glass with spacers and sealant) was invented by Thomas Stetson of New York in 1865, 1895, or 1925?

Of the four types of mortar — M, S, N, and O — M and S are usually used for load-bearing masonry and new construction where high strength is required. Type O is weak and usually only used in interiors. What is type N commonly used for?