So on my system after boot>>
MDV1_ poke_l($28100) + 0:
FS_DRIVR $10 long ????
FS_DRIVN $14 byte 1
RAM1_ poke_l($28100) + 4:
FS_DRIVR $10 long ????
FS_DRIVN $14 byte 1
MDV2_ poke_l($28100) + 8:
FS_DRIVR $10 long ????
FS_DRIVN $14 byte 2
FS_DRIVN is the drive number. So in the third example the 2 in FS_DRIVN, is for MDV 2
As a matter of interest, TK2 seems to redefine,
original ROM
FS_MNAME $16 word+10 bytes medium name
to
FS_MNAME $16 10 bytes+ word medium name + random number
Also I/O and other routines are redefined as the do not point to ROM from the MDV linkage block.
I don't think TK2 changed FS_MNAME. I think it's mistakes in the documentation. It may be that when some of the technical books were published, they were working on pre-release information while QDOS was still changing. I've noticed a few differences between what the QL Advanced User Guide, and The Sinclair QDOS Companion says. And what you actually find.
TK2 alters the linkage block routines because it adds extra features to the mdv device driver (Rename comes to mind). I doubt it actually replaces the mdv device driver completely, Just calls the ROM routines when when one of the extra features is not required.
Martin Head