I'd like to add this feature to my panel (like Kal only not as an afterthought). I found two models on ebay - MK2P-I ($5) and MK2P-NS ($17). Based on this, it looks like the only difference is that the cheaper version doesn't include the test button and LED indicator.
Can anyone comment on the usefulness of the extra features? The price isn't that much different but every dollar saved goes to other upgrades!
On a side note, pioneer breaker has the relay and socket for $8. If I can find a few other things to make shipping worthwhile, I guess my questions are moot since they have the one Kal used on sale.