Edwards was in Deer Lodge Prison, Montana, from 1956 to 1959.
Edwards met Frank Dryman at Deer Lodge Prison.
The Zodiac Killer claimed to have been in Deer Lodge.
For background information see Old Prison Museum in Deer Lodge, especially the 2nd page:
On April 16, 1959, the prison suffered a major riot when two inmates by the names of Jerry Myles and Lee Smart, Jr. led some 12 inmates in an escape attempt. In the melee, Deputy Warden Theodore Rothe was shot and killed, and Warden Powell was temporarily held hostage.
The hostages were held for three days while the riot raged on. After the National Guard was brought in, the two ringleaders died in a murder-suicide, when Myles shot Smart and then turned the gun on himself.