I have exactly the same guitar. Bought it in 1981 in Haarlem, Pays-Bas, for about 330 euro's (including the case). Loved it from the very first moment, and still do. It is very playable for a 12-string, and has this great tingling sould for which we all love the 12-strings. I have been using D'Addario J-38 strings on it for the past 30 years, and intend to do so for the rest of my life.
Keep proper care of this baby. Adjust the truss-rod for playability, don't put the guitar beside the central heating, add some linseed oil to the fretboard once every year, use a bone saddle and nut. That should be it.
Yes, the Takamine F400 may well come from the same factory. Visibly I see no differences.
Rokkomann is from a well-respected Japanese instrument company, back in the time when they created good instruments for the instrument market. It was only later (the 1970's) when other Japanese companies started to flood the market with cheap copies. This Rokkomann/Takamine producer, however, kept producing quality guitars, like few other Japanese producers.
Enjoy the guitar, and please enjoy my YouTube video of it:
http://bit.ly/gPaJrW