Question: A couple of questions:

1) It looks to me that the three-prong ground connection in the bottom right of this schematic is the only ground connection. I used the GNDPWR symbol in my KiCad schematic to represent this ground connection. Can (or should?) this GNDPWR be connected to the "normal" GND which I am using on the jack sockets which connect to the inputs/outputs?

2) Why is the -12v connected directly to this GND? Is this just saying that the GND reference is -12v? But because I am using op-amps and a dual-rail powersupply, don't I also need a 0V GND? Won't it cause a short-circuit if I connected the -12v GND to the 0v GND? I do not understand.

3) Can I use any op-amp for U1-U4?

4) For the op-amps should I use a +/-15V power rails? Or should I use +/-12V?

5) What does the test point 4 mean (near the +12v)? Should I just ignore that?

[![enter image description here](https://i.stack.imgur.com/6CLXU.jpg)](https://i.stack.imgur.com/6CLXU.jpg)

Answer: 
1 and 2) I read that as chassis ground symbol, so negative supply pin is just connected to a metal chassis. This and the fact that the op amps do not use ground as reference but it is biased to half of the input supply pins would suggest that this is a single supply device so it needs single 12V supply and 0V ground, not dual +/- 12V supplies.

3) In general, no. You would have to know what this filter is for and what parameters of the op-amp are important for the circuit operation. If this is from the ARRL handbook, then this is for audio I suppose and it uses LM324 which is pretty generic. And pin numbering must match, but in general quad op-amps have matching pinouts these days.

4) It says 12V on the circuit, not 15V. And based on 1&2, this circuit needs +12V only, not -12v.

5) They are not test points, it just shows the supply input connections to op-amp supply pins 4 and 11, but here the label 11 is missing.
The most important assumption not shown is what voltages are used by the OA V+,V-.

The certainty is that the midpoint between the external , DC V+,V- inputs shown will become the DC output of all Op Amps with a null input. The input and the notch out are AC coupled, the latter of which is dubious for the reason of blocking DC only on this output.

The missing assumptions for 0V and OA Supply and lack of specs , makes this schematic incomplete.