The 'A' model is the latest and greatest version, which the manufacturer introduced a few years ago. Most people want the newest and best filter so they just get this filter regardless of whether they have MTBE. If you don't have MTBE, the filter will last a little longer. To test whether you have MTBE, you would need a professional test from a local lab. If you're on city water, your city water report would disclose any major problems that they know of.
Here is more information on whether you have MTBE and how to test for it:
MTBE in Drinking Water | Drinking Water Contaminants | Safewater | Water | US EPA.
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