newfs_msdos behaviour change between 4.x and 5?

[Geoff Buckingham]

Under 4.x the following produces a bootable dos fat partition:

newfs_msdos -B /boot/win28.b -S 512 -r 32 -i l -c 32 -n 2 -k 6 -m 0xf8 /dev/da0s1
mount_msdos -l /dev/da0s1 /dest
cp IO.SY /dest/
cp MSDOS.SYS /dest/
cp COMMAND.COM /dest/

Where /boot/win28.b is a dd'd copy of the first 28 512 byte 
sectors of a bootable win95 sr2 partition and IO.SYS,
MSDOS.SYS and COMMAND.COM come from the same FS.

(Obviosley the da0s1 need to be created matching the BIOS's 
idea of the GEOMTREY)

However under 5.2.1 the above produces a non-bootable fs

Any ideas where I should be looking for the cause of this?