My memory was faulty – the relationship is rather more tenuous. Here’s the info from Red Rum’s entry in Timeform Chasers and Hurdlers 1975/6, the first ever edition.
Mared, by the sprinter Magic Red,… is out of a half sister to Spring Offensive, who is the dam of many winners including Faberge II, sire of Rheingold.
So Red Rum and Rheingold had the same great grand-dam, a 1934 mare called Batika.
AP