Armstrong was one of the brands made by Raleigh of England (after 1960 in regard to your bike). Hercules, Robin hood, and a few others are the same bikes, good bikes just a notch lower than Raleigh quality. I'm guessing yours is a late 60's. You can find a date code in 2 digits on the rear hub shell. Usually a month and year of manufacture of the hub, which should be almost the same as the bike, 11 71 would be Nov 1971.