[What this symbol means?]

What is this cirquit designed for? In the parts-list is it mentioned?

[Delay introduced by amplifiers]

What you need is Dual Channel Oscilloscope.
Put exactly the same signal in the inputs of the amplifiers (sine wave is preffered), and connect the oscilloscope to the outputs of the amplifiers by two at a time. If there is a delay you will see it. In Higher frequencies will be more obvious.