Definitely a late 1950s or early 60s rifle. Sure there is no date stamp?
They were also sold in the US as a Peerless 35, a Hy-Score 809, and - briefly - as the Beeman 200. And in many places, especially the "British Commonwealth" as a Gecado or Geco. Some of the later ones, at least, must also have been sold as RWS, as well as Diana and Original.