The flat London pattern turnscrew continued to be made well into the 20th century - the military were still using them when I was working as a civilian in an army repair shop in 2002 - stores still had them in stock, but many were stamped 1940's.
For some more info see: http://progress-is-fine.blogspot.co.uk/2012/12/oval-handled-flat-london-pattern.html
I had no idea they were in production so long, although I have one stamped Stanley. However, these were the standard for the 19th Century. Billman, what would you think to be the date of inception? Probably lost in the mists of time, along with the origin of the screw itself. I know I've seen 18th Century examples.
Edkat3, I suspect, like Anglesmith, that yours has been made from an old file. The pattern of the edges of the blade are quite unusual. There were a couple on eBay not long ago, definitely made from old files, with an equally uncommon, but different pattern. With one off examples, dating is the next best thing to impossible.