Richdel, Lawn Genie and Irritrol produced millions of those four valves using dozens of different model number to designate the same valve. Then they went ahead and made more of them that were marketed as Nelson valves with different model numbers.

The 2600 jartop angle valve is made in only thread configuration (no slip) and comes with or without flow control. To determine if it has flow control, look at the top cover to see if it has only one small hole or two small holes in the cover, not counting the hole where the solenoid screws into the cover. If you notice only one small hole with a small plastic (usually black or grey) screw protruding from it, this is the bleed screw. It is used to manually turn your valve off and on. If there isn't any other small hole, you do not have a solenoid valve with flow control option. If there is another small hole with an adjustment fixture, you have a valve with flow control option.

The 2400 Valve is exactly the same basic model as the 2600 Angle Valve, except water enters one side and goes through and out the other side. If you have already determined that your valve has a screw-on (no stainless screws) top rather than one fastened onto the body with 8 easily visible (from the top) stainless steel screws, then you obviously have a 2400 series valve and can go ahead and check to see if it has the flow control option, using the above procedure to do that.

Again, we are assuming that you do have either Richdel or Irritrol Valves.

The above procedures will determine model type if you are dealing with a jartop type (screw-on top) valve only. They are not applicable to models with screws holding the top onto the valve body. We will now get into the other type valves with screws holding them together.

Valves with covers held on with 8 screws - Irritrol makes two basic models that look a lot alike, but have very different diaphragms, so you want to be careful not to confuse these two models. The older model, but still in manufacture, is the famous and venerable 205 model. The 205 is the only Irritrol or Richdel or Lawn Genie residential valve that has 8 holes around the outer perimeter of the diaphragm which are there to accomodate the 8 stainless steel screws that go down through the diaphragm to hold the valve cover onto the valve body and to hold the diaphragm in place. 

Now the reason we need to be careful in identifying which of the two models you have is because both models have 8 screws visible from the outside top. The major difference is the newer 2500 model has a smaller diaphragm without any holes around the perimeter, and the two diaphragms are NOT interchangeable. Obviously once you take the top off the valve, you will know which model it is, because either the diaphragm has holes in it (205), or it doesn't (2500).

We are going to show you how to tell the difference without taking the valve apart, so you can order your diaphragms in advance. An easy way to identify the 205 is examine how the solenoid screws into it's cover port. The cover port on the 205 is molded to be only about seven-eigths of an inch high when it's height is measured at the back of the solenoid port closest to the discharge side of the valve.

Conversely, the 2500 model has a port height of one and one-eigth inch when measured at the same location. The 205 also has the wider diaphragm that can actually be seen located between the cover and body where it is squeezed by those 8 screws to make the valve watertight. The 2500 conversely, has a cover that has a lip that actually fits down over the body about five-eigths of an inch below the top edge of the cover. The 2500 also has a one-eigth inch square post (half-inch high) molded into the cover itself on the inlet side of the solenoid port.  A 205 has no one-eigth inch post at all.

While we are talking diaphragm, let's also mention that a 205 diaphragm does have the screw holes in it and does have a little tit sticking out from one edge. That little tit needs to be located at the solenoid port when you place the diaphragm on the body of the valve for assembly, or the diaphragm holes will not match the holes in the body and when you jam your screws through and tighten up, you will end up with a leak. So make sure your holes in the diaphragm are matched perfectly with the holes in the body before replacing the cover and tightening down screws. Tighten your screws like the nuts on a car wheel, do one on this side, then one on the other side, tightening gradually until the cover sits right and tightens down evenly.

And don't forget the spring.

We have covered specifically Richdel and Irritrol valves, but many Nelson, Lawn Genie and Rain Jet valves will accept the 205 diaphragm. If your Nelson, Lawn Genie or Rain Jet valve has a cover that is fastened on with eight screws, the chances are good, real good, that a 205 diaphragm will fit.

Why is Flow Control important and what does it do?

Flow Control is simply a device within the valve that dictates how fast (or slowly) the valve opens and closes. In areas where ambient water pressure is high, contractors will often install valves with flow control in order to prevent hydraulic ram or "water hammer". When a valve opens too quickly in a higher pressure situation, the water rushes out into the piping and compresses the air in front of it. It is this compression that causes hydraulic ram and results in shaking or chattering sounds in the piping. If this is a chronic problem it can cause damage to the piping of the system and is highly unpleasant to lisen to each time the system comes on in the morning.

Water hammer occurs more frequently and intensely if the piping in the system has not been sized to correct plumbing (hydraulic) theory. If all one size pipe is used in a system instead of sizing correctly, the system is more likely to develop water hammer. Installation of a flow control valve will often eliminate the problem as the valve is adjustable as far as how slowly the valve opens and closes, while the volume going to the system is not affected.