Barrel Marking No4 Mk1*

[12Bore]

Both of the U marks are SA marks. The arrow within the U is the Union of South Africa mark which would have been stamped pre-1961. When South Africa left the commonwealth in 1961the M within the U mark appears for the Republic of South Africa.
Here is a good link

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I had 400 No.4's as surplused from South African War Reserve Stores in the early 1990's.

Perhaps we have one of yours! Are you sure you don't have another you are willing to part with?