I am not sure it matters. I use Constants.FV_SaveFmtInterchange which just uses the current version of FrameMaker to save as MIF. The only reason that version 7 is significant is because it is the last pre-unicode version of FrameMaker and probably uses a different character set. I can't think of a reason that you would need to specify any other MIF version.
We exchange FrameMaker documents with a large number of translators. Some translators use older versions of translation software which do not accept MIF 12 files. Specifically, Trados 2011 needs MIF 11 or below. I'd rather not convert every document to MIF 7, to prevent character conversion issues.
I could try editing the first line of a MIF 12 file to change the version number to 11, and see if Trados 2011 reads those files correctly.
I understand. I just tested this with FrameMaker 2015 and the only values that work are FV_SaveFmtInterchange and FV_SaveFmtInterchange70. Interestingly, FV_SaveFmtInterchange130 also works. It looks like you can use the number of the current FrameMaker version you are using, but there is no reason to since you can just use FV_SaveFmtInterchange.
I suspect that you can simply change the version number and Trados will be fine with it. If you have a lot of files and you want to script this, please let me know. Thanks. -Rick