State of the art, for 1950's. With computers, it got big during the late 80's and you had TEMPEST. CRTs were popular for van Eck because they were essentially small TV stations.
All electronics outside of a Faraday cage leak electromagnetic noise. It'd be relatively trivial to build a jammer and incorporate it into any electronic device, or make a handheld version. At the microvoltage levels we're talking about, FCC isn't going to care even if it technically a violation of the rules. This will also only work for electronics that aren't designed to minimize EMI production, which a lot of crappier designed stuff isn't. So, it'll probably be easy as hell for Widget XYZ and virtually impossible for Widget ABC.