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I think from memory that anything CS with a model number ending in 1 indicates an ink pencil.
Close, John, but no cigar! The '1' should be at the other end, in that most model numbers in the range 100 - 199 were ink pencils, until they started filling in spaces in the number book more randomly, from the 1950s onwards. Being CS, of course, there are plenty of exceptions, not only the 81 Dandy Ink pencil and the 91 Universal ink pencil, as John mentions, but also an 82, 92 & 93.
I can't add to the useful practical information that John gave, but one thing I do find interesting is the original price difference between nibbed pens and ink pencils. In 1924, around the time of launch, a Pixie nibbed pen cost 7/6, whereas the Pixie ink pencil cost only 3/9. Similarly, the first Dandy nibbed pen cost 6/6, while the Dandy ink pencil was only 2/9. These prices presumably reflect the price of gold at the time, a point reinforced by the huge differential in prices of essentially identical pens with and without rolled gold trim, for instance the Dinkie 540 (no trim), at 5/6, and the Dinkie 526 (2 broad cap bands), at 7/6.
Andy