Complex sentence: Will Travel was very similar to the backup Oxygen Purge System on the real Apollo Primary Life Support System, the only major difference being that the Apollo suits had a largely automatic pressure regulator, and Heinlein's suit had manual pressure regulation.
Split into simpler sentences: Will Travel was very similar to the backup Oxygen Purge System on the real Apollo Primary Life Support System. The only major difference being that the Apollo suits had a largely automatic pressure regulator, and Heinlein's suit had manual pressure regulation.