/usr/local/lsws/conf/httpd_config.conf is the main configure file.
it includes httpd-vhosts.conf, which include /usr/local/lsws/conf/httpd-alias.conf,
ips.conf: include /usr/local/lsws/conf/httpd-alias.conf as well.
How did you downgrade OLS from 1.7.3 to 1.6.15? Some downgrade/upgrade method won't change VERSION file and only replace the binary file.
https://openlitespeed.org/kb/how-to-upgrade-openlitespeed/
You can always get the correct version by running
/usr/local/lsws/bin/lshttpd -v
CUSTOM...