From the Lawrence Daily Journal-World for May 20, 1975:

About 8,000 names were to be removed from the list of Douglas County’s registered voters. About that number of letters had been returned undelivered to the county clerk’s office out of approximately 13,000 letters send to registered voters who had not voted in the November general election, for which registration had totaled 35,938. The returned letters reflected the number of non-voters who had left town or changed addresses. Because state law required that voters re-register when changing addresses, the names were to be deleted from the registration list. State law required such a purge after each general election. County Clerk Delbert Mathia was a little puzzled at the large number, saying he had expected the returns to total only about 4,000.