Are there any fbus specifications available?

The only useful documentation if found is the one from gammu/gnokii which can be found on my github repository for example:

nokia.rst - The general nokia part which covers fbus and mbus
n6110.rst - Nokia 6110 and derivatives (Nokia 6130, 6150, 6190, 5110, 5130, 5150, 5190, 3210, 3310)
n7110.rst - Nokia 6210 and derivatives (7110)
n6510.rst - Nokia 6510 and derivatives (?)

The rest of the documentation can be found here.

If there is more documentation out there some one may can drop me a comment.

Some more c++ code can be found here.