SOLVED - it is at Webmin -> Servers -> BIND DNS Server -> Module Config -> Zone file options -> Format for the name of forward zone files. "Module Config" is the little gear icon in the top left corner left of the "BIND DNS Server" title at the top [per as answered by author - J. Cameron and put here for benefit of others searching for same in the future]
SOLVED - it is at Webmin -> Servers -> BIND DNS Server -> Module Config -> Zone file options -> Format for the name of forward zone files. "Module Config" is the little gear icon in the top left corner left of the "BIND DNS Server" title at the top [per as answered by author - J. Cameron and put here for benefit of others searching for same in the future]
Centos7