[Arran]

 Philco did not seem to go for the idea of building any true "Hot Chassis" sets, they almost invariably used a capacitor like a .22 uf in parallel with a 220,000 ohm resistor tied between the B- line and the chassis, the older ones will have a crude RF choke in series with the .22 uf as well. Here is something interesting to try, when you say that you don't feel a current when the set is plugged in a certain way, and the radio is on, try touching the chassis when the radio is turned off. I would be willing to bet that you will feel a little bit of current whilst the set is turned off if there is no current whilst the set is turned off. So basically you can take you pick, do you prefer a buzz from the chassis whilst the radio is on or whilst it is off? Actually since the chassis is inside a wood or plastic cabinet, with plastic knobs nobody will be normally making contact with the chassis.  So this is why I don't totally buy into the polarized plug idea, when it comes to AC/DC radios there is really no way to make them 100% safe short of running them through an isolation transformer, only ways to make them less dangerous. Another more economical suggestion would be to operate them through a GFI outlet, like the use in bathrooms, that way if you end up with a ground fault situation, the breaker will trip instantly.
Regards
Arran