I think the Norman Ough plans ended up in the David MacGregor plans collection, I have an old catalogue listing these, the Tribal drawing was of Matebele rather than Cossack.
The MacGregor plans are currently in the possession of the Great Britain Trust at Bristol and many of them are reportedly in a fragile state. Despite the Trust having said they would be available again ‘soon’ for some years, recent enquiries have given no indication that they will be on sale again anytime in the near future which is rather frustrating.
Colin