Curious. I have a larger one like this: 30×20cm, also with a flat lever key. And another, same size, all black, with a 3lever pin key. They both have the same action: latching when closed, and deadlocking with a turn of the key. Neither has a coin tray insert, nor supports for a tray.
The boxes are strongly made of pressed steel, unlike the common soldered tinplate cashboxes.
I cannot measure the thread guages but they measure more neatly in cm rather than inches. The flat steel key suggests America, whilst the elaborate paint job suggest late 19C. Plain black I guessed more likely 1920s. The latching-deadlock function is not, afaik, typically British.
So where from, and when? My guess is European, possibly German, and being pressed, maybe 1920s? But would Germany have been making anything like this in the 1920s?
As a matter of curiosity, Hitler’s dispatch box is also strong, nicely painted, but square-cornered, with a pin key lock.
https://www.alamy.com/stock-image-despatch-box-used-by-adolf-hitler-removed-from-the-bunker-beneath-165991873.html This is a large file, so this is a link 60 MB (1.8 MB Compressed download)
There is a 3D animation here https://sketchfab.com/3d-models/hitlers-despatch-box-998a58802547434bb67e091379b16e73
Despatch box used by Adolf Hitler. Removed from the Bunker beneath the Chancellery in Berlin by Lieutenant Colonel I F Anderson OBE MC, Seaforth Highlanders in May 1945. Part of the Highlander’s Museum collection (Queen’s Own Highlanders Collection).
The Highlanders’ Museum, Fort George, Ardersier, Inverness, IV2 7TD.
I look forward to any information on these cash? boxes any can provide.