Back in the 1960s, Space Jet Bell made inexpensive electro-mechanical slots to compete with Bally. Their machines were fun to play and most had a 50 coin, tic-tac-toe mini jackpot with 1 in 148 odds that made them very popular. Unfortunately, the brain box that calculated payouts for the coin hopper was poorly designed, broke down often and was a nightmare to maintain.
This Craigslist ad is for a piggyback Space Jet Bell, containing two mechanisms in one case and requiring two coins to play. I have never seen one before by this manufacturer. I know that Harrah's Casino custom built piggyback mechanical Pace machines and Harvey's did something similar with mechanical open fronts back in the 1950s, but I never saw a commercially built piggyback before.
https://mohave.craigslist.org/for/6062166256.html