Why are some chainrings specs appended with an 'a' or 's'
From this article:


Type B: by far the most popular one and also the default on Ultegra
cranksets; it comes in 52 and 53 teeth and must be paired with 39
tooth inner chainring.
Type A: comes in 53 as well as 54 and 55 teeth; as per Shimano’s
advice must be installed together with a 42 small ring.
Type E: a VERY rare combination: 56/44


There is NOTHING unusual about the 39, 42, and 44 tooth chainrings. It is the outer big ring that has special gates and pins machined into it for smooth and fast shifting. Since the 53/39 cranksets are dime and a dozen, flatlanders and time-trialists alike may opt for a 42 inner ring for tighter gearing. The more inquisitive like myself are aware of the A and B type Shimano story so we go on Google and search if it is possible to mix and match. And what do you know, the internet says it is, so it has to be true!



As for the 'S', Shimano is Japanese and using an S is very common from Japanese manufacturers to denote a newer or Superior product. Nikon does this with their cameras, Shimano does it with chainrings and component updates, and so on. Shimano has also used S to denote a product is a sprocket/cog and many other manufacturers- some Japanese, some not- use an S to mean single speed.