The book ‘Deigma: A First Greek Book’ by Walters and Conway (1916, 1929 etc) is available on the Internet Archive at https://archive.org/details/cu31924052695222
I recently found the Key to this course in a second-hand bookshop (published 1920), and have uploaded this to the Internet Archive as well, at: https://archive.org/details/DeigmaKeyWaltersAndConway
It contains pretty thorough corrections to most of the exercises in the original book.
Thank you. This is comprehensible input for me and it was completely unknown to me before now.