Hello JoyJones, what a beautiful latch!
Although I am familiar with many of Cotterill’s distinctive key locks I have never seen one of those, it looks to be a heavy duty Cotterill version of the type often referred to as a “Suffolk Latch”.
When you discovered it during renovations was it actually fitted to one of your doors?
It would be great to see a photo of the other side if you are able to take one, it might also help anyone who has any information on it for you. What a great find!