Their
http://hdl.handle.net links are supposed to work, and to let them change their local address for items and redirect to the new address via a permanent handle. But it doesn't seem to point anywhere existent at the moment (at least for many people including me). Currently working links for the Smithsonian items are:
Hough:
https://repository.si.edu/handle/10088/13062
Eggers:
https://repository.si.edu/handle/10088/13071
Means:
https://repository.si.edu/handle/10088/15071
Trousdale:
https://repository.si.edu/handle/10088/1332
Krieger:
https://repository.si.edu/handle/10088/10135