I just went through this entire thing myself and want to help others avoid the same rabbit hole. To answer the original question.....The original Neptune OS-1 sensor does NOT fit into the KLIR V1 controller. I tried that yesterday, as my KLIR V1 roller has a bad level sensor, and I had several of the Neptune level sensors. The jack plug on the Neptune sensor is bigger than the one on the KLIR. So don't buy a Neptune OS-1 expecting to use it on your KLIR filter roller. I would assume there is an adapter you could probably buy to make it work...but I haven't tried this.
I ended up buying a level sensor from store.coralvue.com. Icecap used the same sensor on a few products. Try searching "Eye Sensor IR for IceCap ATO and Klir." That is the black version. If out of stock, they also make a white one. I confirmed with Coral Vue that the white also works perfectly fine with the Klir V1 roller. $12 sensor is a lot better than $70-80 to upgrade to Klir V2....which I can attest is not better than the V1. Save your money.