I had already gone ahead with sudo bootctl set-default 4acb3ccd578940f4b1d99d71b59ee048*6.6.72-1-lts.conf
command.
I have subsequently reissued that command, but this time with only lts.conf behind the asterisk.
On the black systemd-boot screen the highlighted line with the little arrow was still show as 6.6.72, presumably because that is the current LTS kernel. Presumably when that changes it will change automatically in the boot screen because of my *lts.conf.