1940s British.
When looking (say, on eBay) if it says "Made in England" on the back and "Ever Ready British Made" on the inside of the baseplate then chances are it'll be the right one. While there are some pre-WW2 examples, British production was mostly post-WW2. Production did continue into the 1950s, but again ... most was 1940s.
That's the one ...
Why? The plating is really good and you're likely to pick up one (any one) which will clean up near perfectly and be a great example for you. Clean up? Yeah, toothbrush and liquid soap. That's it! I've cleaned up scores this way. Only on occasion I've had to bust out the Peek polish; same drill, toothbrush, scrub, then liquid soap onto the mess, scrub and hot water. Gleaming!
If you really get into these, yeah, the Streamline is reputed to be the most desirable but if you google "Sun Ray 1912" you'll see a real beauty! The Streamline is the Rolls Royce, but the Sun Ray is the E Type.
Alternatively, look for a gold tone American. Chances are the gold tone will have washed off already and little more than a good scrub with a toothbrush and Brasso will bring up an all brass example just lovely.
Happy hunting!