The book 'Latin prose exercises based upon Caesar's Gallic war' (1884) by Clement Bryans is available on the Internet Archive:
I have just uploaded a scan of my copy of the KEY to this work:
https://archive.org/details/KeyToLatinP ... esarBryans
However note that this is the key to the second edition of the Bryans work, and not the first as available on Internet Archive. ...