Found the "diamond inside a U" :

"Sometime after WWII a diamond makes it's appearance within the South African U. It is believed that this mark was used after 31st May, 1961, when the Union of South Africa left the Britishicon Commonwealth and became the Republic of South Africa. This mark has been seen on both rifles and oilers".

Government*Ownership*Marks

Strange that a 1900 Long Lee should have a post 1961 South African mark !