There isn't actually a 3.9 release of JMRI, and unless they change their version numbering scheme (not likely), there never will be.
The convention JMRI follows for numbering releases is R.n for Production (stable) releases, where "R" is the release number and "n" is the version number, which is always an even number for Production releases. For example, 3.8 is the latest Production release, and 3.6 was the prior Production release.
For test releases, the convention is R.n.x. The "R" and "n" are the same as above, except that "n" is always an odd version number denoting a Test version. The "x" is the minor (point) version number as improvements/fixes are made to the test version.
There are RARE instances where a major bug is found after a Production version is released, in which case a R.n.x Production version may be released. However, in my ten-plus years of using JMRI, I recall only a couple of those instances ever happening.
Having said all that, MRC support was first included in Test release 3.9.1, and I believe some updates were done to that support in 3.9.2, which is the most current Test release.
Edit: Fixed some grammar and punctuation.
JMRI version 3.9 is the test version with MRC support.
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