To add to what mrfields said above Benchmade uses letters with their model numbers to denote certain features. Models with “S” have blade serrations. So the base model you’re inquiring about here is a 3550, also known as the Stimulus, a push-button automatic knife designed by Mel Pardue with a black coated blade (the BT part of the model. A 3550BTS or 3550BKS designated model would have a black blade with serrations. A 3550S would have serrations, but an uncoated steel blade. Additionally, most manual (non-automatic) models have a three number model, whereas autos have four, such as your example. “LE” in some models denotes Limited Edition.
You might see models with “dash” numbers, like 940-1. These are usually variations of existing models with different blade steels and handle materials than the original model came with.
It can be confusing at first, but it starts to make sense after a while.
I believe the BT was a Teflon based coating, but could be wrong.