Both were used since 1966. There is no particular time period during which one or the other was used.
The surface of the embossed cameo is raised. Although both show an impression on the back of the stamp and the flat printing is somewhat raised as well, the difference is easily visible.
At the top is the embossed cameo on a 1966 Christmas stamp, at the bottom the flat cameo on one of the two 1966 'Battle of Hastings' stamps.