Could that have been the main when power was first installed and then when power was upgraded (there clearly is more than 70 amps of power being used thru that entire panel), leaving that main only controlling the few original breakers?Since it is labeled MAIN it is, at least, likely that it is the Main Breaker that controls all of the power to that panel. If that panel is supplied from the electrical meter and not from another panel, switch, or fused pull out it is the "Service Disconnecting Means" for the home.
We could be more sure of it's function if you do what Snoonyb suggested and remove the dead front from the panel's cabinet; DANGER: Removing the front cover; which is formally called the "Dead Front" exposes energised parts. Contact with those parts will cause imediate severe injury or death; then photograph the interior, and post that picture here. If the panel is not mounted in a finished wall; and you can see the cables that are connected to it; it would be helpful to also post a picture of the exterior view showing the cables that are attached to the panel's cabinet.
I would read this link and if there is any way possible replace that box at once. Zinsco boxes are outdated and have serious issues. To not replace the box is putting yourself and your family in danger.