Um, the top cover is secured by the three quarter-inch hex-head bolts on each side, one below each rivet.
The rivets serve to secure a metal strip to the top cover. This strip extends below the lower edge of the top cover and serves to anchor the three hex-head sheet-metal screws that hold the case halves together.
Remove those six screws and the top cover comes off. Remove the four screws on the underside and the lower half of the cabinet comes off.
Unless you fetched it out of a time machine fresh from 1977, there will be age issues as a rule. Removing the bottom cover will permit you to check for visible signs of impending doom, like swollen electrolytic capacitors. If yours has the white PVC-plastic 1000uf 25-Volt axial-lead capacitors (one wire out each end) make sure none of them are beginning to swell up. This is the first sign that the part is going to short out and cause mayhem some time soon.
With any luck, you'll hear similar advice from other folks about 45 year-old electrolytic capacitors and how they tend to fail soon after they are awakened from a long sleep. Doesn't mean it won't appear to work fine as you found it. But the statistical reality doesn't lean in your favor for the long run.
You have a rare find here. Would be a shame to see it go poof in a week or three because of a predictable issue.
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