Thanks LabelKing and Tarmac. LabeKing - this Seiko is from the 68-71 era, hence the style cues you've identified. Tarmac - 'DX' means, or meant, 'deluxe' - the movement is the 'Seiko 5' base level movement, but the case and dials of 'DX's were of a higher grade and finish. In the old catalogs DXs were one level above the entry level stuff. The second highest level was 'King Seiko' and the ultimate being 'Grand Seiko'. Grand Seikos are still being made - Japan domestic market only, although anyone can order them from dealers over there.
Cool Seiko, I have a similar one: My first proper watch at age eight, except a Timex which went to shit quite quickly. I have an uncle who was a lot in Japan in the seventies, when things like watches, optics and cameras were still very cheap there. I'm having it pimped now, with an after-market, sterile diver-style dial, matching new hands and a complete overhaul. Not really worth it, but it has some emotional value.