A document prepared for the New Hampshire Division of Historical Resources is a 152 page report on Lean-tos and shelters in the Whites. It has area maps and historic photos and it is a wealth of historic information.
Check it out at fs.usda.gov/Internet/FSE_DOCUMENTS/fseprd649771.pdf?fbclid=IwAR0FNhd8rEcZ4JJJaijcFV5BfMqQoUjoLdBi-aU15yYe75VmMmt0XZqNcps
Here is a shot of Imp Shelter found between 1938 and 1980, where I camped in 1974.
Enjoy!