Interarms Mark X rifles were imported from Yugoslavia (now Serbia) and sold in several grades/configurations, one of which did indeed have a Mannlicher stock, called the Continental IIRC. The 375 H&H was not offered in a Mannlicher stock, it was offered in the Viscount, Alaskan, African, and Whitworth series, all of which had a half stock, pistol grip, black plastic forend cap, and 20 lpi checkering. The Whitworth was probably the nicest, but was still fairly plain. Your appears to be a custom due to the lack of checkering and the Mannlicher stock.