The Coventry and District Permanent Money Society is listed on the Financial Service Authority's Mutuals Register as having been registered under the Friendly Societies Act 1974, but they deregistered on 25th July 1974, which would seem to indicate that they were wound up at that time.
If I remember, a six penny stamp would indicate that this document is a contract, whereas a receipt would have a two penny stamp on it, but I could be incorrect.
From what you say about the information on the back of the certificate, I would think that your Grandfather and the four others may have bought shares to a nominal value of £50 in the society, paying a first tranche of 21%. They may have been called upon to pay further tranches at a later date, although the 2nd World War may have put paid to this process, especially with the bombing of Coventry.
From 1 December 2001 the FSA took over the functions of the Central Office of the former Registry of Friendly Societies, but they do not seem to have made much progress with copying old documents, but if they hold any information this would be quite expensive to obtain.
I would be inclined to regard this document as having no value other than sentimental.