Hmm. OK, I tried putting
[archive]
path = /mnt/user/archive
comment =
browseable = yes
writeable = no
read list =
write list = stephen,nigel
valid users = stephen,nigel
case sensitive = auto
preserve case = yes
short preserve case = yes
vfs objects = catia fruit streams_xattr worm
fruit:encoding = native
worm:grace_period = 86400 # 1 day
Into the SMB Extras, but the samba configuration file is unmodified.