My .375 is SN B2704** and was purchased new in 1984. The actions and barrels were made by Zastava in the former Yugoslavia. The Whitworth rifles were assembled in England, while other Mark X rifles were mostly assembled in Yugo but suspect some were also assembled in England, but its hard to definitively state or prove this.
Very early Whitworth Express rifles will show a stamping of the Interarms castle on the barrel just ahead of the receiver and will say as example .375 H&H Calibre instead of caliber. They will also be stamped Whitworth on the barrel and the top of the left side rail.
Believe they were first introduced in 1974.
Hope that helps.
Here is mine still looking good after 3 safari's.
