How can I tell if this is a split bus panel with no main breaker?
The arrangement of the breakers looks like my 50-year-old GE split bus panel. With the dead front off I can see the wires that go from the lower right hand one of the four 2-pole breakers at the top to the lower bus. In my panel the lower right 2-pole breaker is a 60-A breaker which feeds the lower bus for all the 1-pole breakers below.

In your panel it looks to me like the lower left 2-pole breaker has two heavy wires which "disappear" under the 1-pole breakers and so may connect to the lower bus. If you switch off this breaker, does this cut power to all the 120-V circuits?

What are the four 2-pole breakers powering? In mine they are (CW from lower left) electric dryer 30 A, a/c condensing unit 40 A, electric range 50 A and "main" 60-A feeding the 1-pole breakers below.