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: 
I suspect that this circuit is intended to use a single 12 volt power supply, with the negative terminal of the supply connected to the circuit ground. The power input labelling is misleading - it appears to imply a total 24 volt supply (+12 and -12) rather than a single 12 volt supply.

Test point 4 and the test point connected to Ground are just handy spots to connect your meter to measure the supply voltage - you can ignore them.

Does the place you found this circuit recommend any particular op-amp? If not, I expect any op-amp that will work from 12 volts will do.

When you want to ask questions about a circuit you found somewhere, you should include links to the source (and you should look around the source - it might answer your questions...)
I breadboarded this up and after a couple false starts, I finally have some answers:

1) Connecting -12v and +12v causes it to start smoking. So the schematic is wrong labeling it as -12v. Don't do this.

2) Connecting it up with +12v and 0v (GND) works perfectly! They should remove the -12v label as it is simply incorrect, it is actually just 0V and GND, NOT -12v. This schematic is also missing a label #11 on the lower GND input, which is the negative supply for the op-amp. The op-amps are supplied by pin #4 and #11. I am assuming they incorrectly labeled it -12v because usually the op-amps are dual-rail, but in this case they are powered by just +12v/0v (single supply). The op-amp is the LM324 as indicated on the original schematic and text which can be found at <https://www.americanradiohistory.com/Archive-DX/Ham%20Radio/70s/Ham-Radio-197802.pdf> on page 70 (page 72 in PDF). Thanks to @SamGibson for finding the original schematic.

So for anyone else looking at this schematic, the -12v is wrong and should be marked 0v.

This is a great little filter.