It's not a Raleigh 26 tpi headset, so that's good. Raleighs of the 70s are said to have been fitted with headsets with 30.0mm cups. Before shimming the cup, I'd get an accurate as possible measurement of the cup mating surface and see if it's less than 30.2 mm. If it is, and if the headtube is not out of round or larger than 30.2, I'd buy a modern iso headset and install that sucker. Although the Raleigh headtubes that I've measured have been about 30.0 mm, ISO headsets, like modern Tange Levin, press right in with the interference fit. If you do decide to replace the headset, pop the crown race off the fork and measure the fork surface that will hold the crown race. Lots of Raleighs had 27.0 mm while most would have been 26.4 mm. Headsets like the Tange Levin are available with either 26.4 mm or 27.0 mm crown race dimension. Cups are 30.2 mm (though there is a JIS version with 30.2 mm) with either crown race.
Personally, I heart the Tange Levin headsets. They are cheap but well made, look good on vintage steel, and have a short stack height so it's unlikely to find a fork/headtube that it won't work on. Just add some spacers to make everything happy.