fsck flags for ufs1 on 5.2

[Hanspeter Roth]

Hello,

when I run fsck on a UFS1 which had been created with FreeBSD 4.x on
FreeBSD 5.2 fsck reports an error and wants to fix it.
Later on FreeBSD 4.9 fsck also reports an error:

FREE BLK COUNT(S) WRONG IN SUPERBLOCK
SALVAGE [yn]

How can one make fsck on FreeBSD 5.2 tell how to treat UFS1 in order
to keep FreeBSD 4.9 happy?

How ought a UFS1 entry be made in /etc/fstab on the FreeBSD 5.2 in
order to not disturb 4.9?

-Hanspeter