Thanks for the new serial numbers everyone! Interesting that they all fall within the known serial number range.
Besides the existence/absence of a red C, here's another variation that I noticed recently. Different colour coating!
I did notice a perceptible difference in how they render colour too. The lens on the left renders slightly "warmer" (and perhaps is more suited to B&W) and the lens on the right renders slightly "cooler". Makes sense as a coating change in the late 1950s would probably have been to improve performance for colour film.