The marks indicate the following:
Sponsors (formerly called makers) were CS * FS for Cornelius Saunders & Francis Shepherd.
The had factories in London, Birmingham, Glasgow and Chester at various times. Jackson's describes them as "prolific manufacturers of small work and jewellery."
The large X is the actual maker's tally mark for the journeyman apprenticed to the firm of Saunders & Shepherd.
You have the other marks correctly described.
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