Assume you mean the sleeve on the outside of the analogue board? In a 512K it should be a white vinyl sleeve, unless it was replaced with the older coated cardboard one at some point. You run very little risk operating it temporarily without the sleeve on, however, the sleeve is there to prevent the solder side from coming in contact with the metallic paint on the inside of the bucket and shorting. This could happen if for some reason you pick it up and move it while it was turned on, or otherwise press in on the bucket. I would not recommend operating it in this condition normally.