The C Tube was designed in two different versions, one for submarines and one for chasers or other small craft. The former was called the S. C Tube (Submarine C Tube) and the latter the S.C. C Tube (Sub chaser C Tube). But in most of the literature, the C Tube on chasers is simply called the SC Tube.
Thanks to David Propert for submitting the images to The Subchaser Archives.