You must be speaking of the A/C Interlock. That's where the A/C line disconnects from the chassis when you remove the rear cover. That way you cannot turn it on with the rear cover off. Of course, you can by just making another line cord or removing the interlock from the rear cover.
I would doubt there is any high voltage left stored in that old set... unless someone has turned it on very recently. If it has been on lately, you would just need to use caution around the wire going to the bell of the CRT, and also use caution in the HV cage. Other than those areas, you should be OK.
