Well, for all of the Nexus devices I've owned (Galaxy Nexus, Nexus 5, Nexus 7, Nexus 6P) the build number does indeed correspond to a specific version of Android.
Here's the page for all of the Nexus (and Pixel) devices and their build #s:
This page,
https://source.android.com/source/build-numbers.html, says this:
Starting with Cupcake, individual builds are identified with a short build code, e.g. FRF85B.
The first letter is the code name of the release family, e.g. F is Froyo.
The second letter is a branch code that allows Google to identify the exact code branch that the build was made from, and R is by convention the primary release branch.
The next letter and two digits are a date code. The letter counts quarters, with A being Q1 2009. Therefore, F is Q2 2010. The two digits count days within the quarter, so F85 is June 24 2010.
Finally, the last letter identifies individual versions related to the same date code, sequentially starting with A; A is actually implicit and usually omitted for brevity.
The date code is not guaranteed to be the exact date at which a build was made, and it is common that minor variations added to an existing build re-use the same date code as that existing build.
I haven't checked my non-Nexus devices for their build numbers (I might later and re-post or update this post), but check to see if your's matches the above description. Is "b27" the only string for the build # that you have (or are you referencing something in the Kernel version?).
Cheers!
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) in the various ZTE root threads mention build numbers of that format, but didn't know if it was just a portion or a shorthand for something longer.