Is it possible to find out when a Kc registered dog last sired a litter of puppies ?

As long as they have been registered, yes. To look up the dog on the new KC Health Test Results Finder: Type in the dog's name, and click on his entry when it appears. Scroll down, his litters should be at the bottom of his page, but read carefully - for some reason they seem to be listed in a random order.

Thank you, I will have a try.

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That worked well, thank you.
My suspicion has been answered.