This is a sub panel in my mom's house. It is not the infamous stab lock type. It dates back to the early '50's when the house was built and inspected (note the bonding screw even though it's a subpanel) The main cable is an old style BX without a ground wire, so its resistance back to the service equipment may be too high to take a fault current at 50 amps. Thus I hesitate to remove that screw.
Never saw anywhere else 3 breakers as a single unit not intended for 3 phase service. They work well, once I was using an electric lawn mower outside, and my mom decided to have toast, both on the same 15 amp circuit. The breaker kicked out after a half minute, as I understand it, as designed.
Is it still code to connect a pair of main cables (one of which feeds another subpanel) like this? The inspector okayed it back in the '50's. I'm not inclined to mess with anything in this panel. I suppose it is grandfathered.