I am using a book detailed below to answer this. The C1 to C100 series are shown as private and in-house recordings. C1 was apparently not used and the last in this series was C97. The ones you have are described as "demonstration samplers". There were 38 of these and issued monthly. I presume, although the book does not confirm, that they were issued to record shops to encourage them to stock HMV records. The first of these samplers was C17 issued in January 1932. The last was C78 issued in December 1934.
The book I have used is by Michael Smith and is "A Numerical listing of the 12 inch C Prefixed series.......". This contains matrix numbers, recording dates and other relavent information. It was published in 2001 by City of London Phonograph & Gramophone Society. They also have other very good discographical listings books. See their website www.clpgs.org.uk (There is a bookshop section.)
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