Found this Web Link doing a quick Google search. http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Silvertone%20Radio%20(Parts%20List%...
Not sure if it is the manual for the unit you have.
If the manual is for the unit you have there are a couple of things you can do.
First, you will need a multimeter to measure AC voltage. You will be measuring for the proper AC polarity orientation for the 2 blade, prong, plug on the 2 wire cord and plug.
Plug the Sears 1270 into the wall 120V outlet and turn the unit on.
Set the multimeter to auto AC volts, or an AC volt scale above 125Vac. Touch one test lead probe to a bare screw or the bare metal chassis on the Sears 1270.
Insert the other test lead probe in the equipment ground hole on the wall 120V outlet. Make sure you make a good contact with the equipment ground contact.
Note the AC voltage reading.
Next measure the AC voltage from the wall equipment ground contact to the turntable. The voltage reading should be the same as above. If it is not the same unplug the 120V plug from the Sears 1270 chassis outlet rotate the plug 180 and plug it back in the outlet on the 1270 chassis. Remeasure the voltage again both from the wall equipment ground contact to the chassis of the 1270 and from the wall equipment ground contact to the turntable. Both should measure the same. Note the voltage reading.
Next turn off the 1270. Unplug the 1270 plug from the wall outlet, rotate the plug 180 and plug it back into the wall outlet. Turn on the 1270. Measure the AC voltage again from the wall equipment ground contact to the 1270 chassis.
The lower of the two voltage reading is the correct plug orientation. Mark the plug is some manner.
One plug orientation could have a chassis AC voltage quite a bit higher than the other. Again you want the lower of the two AC voltages.
Second choice is to connect a ground wire from the wall equipment ground contact to the chassis on the Sears 1270. (If the wall outlet is a standard 3 wire grounding type outlet you can use the center 6/32 trim screw that supports the wall cover plate.
The ground wire may add noise to the sound you hear from the 1270. OR it may sound quieter.