There were three distinct eras:

1 - J. B. Miller Keyless Lock Company, Kent. 1888-1944. Your lock would fit this era. The earliest advertisement for this style I've seen is from 1939. So a good guess would be 1939-1944 for your lock. The company was formed and run by James B. Miller until he died in 1927; his sons Ned and Jamie continued the company.

2 - Gougler Keyless Locks, Kent. 1945 until the 1970s (does anybody have a more precise date). It isn't clear to me if the Miller company went bankrupt and Gougler picked it up, of if the Miller sons decided that simply selling it would be a good business move.

3 - Miller Keyless Locks, Columbus. 1970s until perhaps the early 1980s. Again, I'd appreciate more precise dates if anybody knows. This was the Miller family not related to the original Miller family. Again, it's not clear why Gougler let go of their Keyless Locks Division. Perhaps they wanted to concentrate on their other products and the lock division just wasn't a big money maker.

There is possibly a fourth era, between Gougler and the second Miller family. An eBay user "dtwiant" once implied that Gougler sold the lock division to 5 stockholders, who in turn sold it to the second Miller family. I haven't been able to corroborate this in any other source.

I have a large (9 MB) set of notes and images on this, as a Word document, if anybody is interested.