Interesting. If I remember correctly the source code was available (need to check my old disks), however most likely the licence would forbid that anyway.

IIRC Borland released the C++ version specifically as PD later on their FTP server, it isn't based on the version from Turbo C++ physical releases. The history is (very briefly) mentioned in the Free Vision wiki page at the FPC wiki[0] (note that the wiki needs cleanup, e.g. it mentions 64bit clean support as a todo item but FV has been 64bit clean for a very long time now). It also mentions that somewhere between the C++ version and the Pascal conversion, TV/FV was converted to use graphics instead of text mode and it was ported back to text mode -- considering all the conversions, i'm surprised the API remained largely the same so that even now the best way to learn Free Vision is to read Turbo Vision docs/tutorials/books :-P.

[0] https://wiki.freepascal.org/Free_Vision