Chains from that era were pinned, not riveted, and no master link was used because the pins could reliably re-close the chain. As the number of sprockets on the rear cluster increased beyond seven, chains needed narrower width, and the protruding pins on old-style chains interfered with this. Riveted chains were developed to address this, with peened ends on the rivets to hold them in place. Pushing out a rivet destroys that peening, so the rivet cannot reliably re-close the chain after being pushed out. Instead, a master link is used to close the chain.
If your current chain is still good, you can continue to use it without a master link until it wears out. Once that happens, you'll need to replace is with a riveted chain and a master link, as pinned chains for derailleur bicycles are no longer commonly available.