Milner made safes from the mid 1800s and after merging with Chatwood nearly 100 years later, safes were marked Chatwood Milner. Subsequently they were bought by Chubb in 1962. The Chubb company changed hands until becoming part of the Assa Abloy group. Assa kept the door lock side but immediately sold the safe business to Gunnebo who currently use the name on safes under licence from AssaAbloy.
I'm sure your safe will be well over seventy years old. There were some articles about Milner in Tony Beck's Lock Collector newsletters linked elsewhere on this site.