I am probably overthinking this, but I brought my car in for service and they did not screw on one of the gray OEM plastic caps.

I really did not want to drive back again (they messed up my service the first time with incorrect fluid weight so I already had to go back to get that corrected)

So I just went online and bought 1x 40703-JA00A.

After the fact I did a little searching against my year and model and it said that part was not compatible, and the compatible part was 40703-JK00B.

I would think they are all compatible as it is just a valve stem cap.

The 40703-JK00B is $11 more than what I bought 40703-JA00A for.

From pictures they look the same, are there any real differences between these part numbers, maybe different threads or something else? Or are they all the same and just keep the 40703-JA00A even though Infiniti says it is not compatible but 40703-JK00B is.

 

I believe you are overthinking it. It's irritating that they forgot your cap but as you see the cap is not needed to keep the tire pressurized. And in fact any valve stem cap should work fine including a wide variety of plastic or metal caps which can be purchased at any auto store. Even pink ones if you prefer.

I think the part number you referenced above; "40703-JK00B" is actually a deeper part of the valve stem and not related to your missing cap.