Could you point me to a specification for this?

Yep: right here (from memory):

The bitrate tag in the *1 file consists of two letters, followed by a number.

The first letter specifies whether the file is fixed or variable bitrate. It is an 'f' or a 'v' respectively. If the database rebuild code couldn't work out which, it is a '?'. ABR files will appear as 'v'.

The second letter specifies whether the file is stereo or mono. It is an 's', or an 'm'. You might see 'j' for joint stereo, but I think that these files are all labelled with an 's'. If the database rebuild code couldn't work out which, it is a '?'.

The number specifies the bitrate in Kbps. If the database rebuild code couldn't work this out, it either makes up a number (usually 128), or stores a zero, or stores nothing.

Some examples of valid bitrate tags are:

fs128
vs179
fm64