Chivas says : The oldest malt whisky distillery in the Highlands of Scotland.
Main malt in Royal Salute 21Y.O. ???

From the book : 'The Scottish Collection' Classic Malts by Carol P.Shaw
Tasting notes : A big, robust whisky which is full-flavoured and fruty, with a nutty sherried sweetness. Taste rating = 4
According to records, poduction of a 'heather ale' by local clerics had been taking place in this area as early as 1208. The later siting of Milton Distillery (as Strathisla was formerly known) here in the eighteenth century may have been for the same reasons: set in a bood barley-producing area, with easy access to a pure local spring which had been a holy well of local Cistercian monks, and said to be guarded by water spirits. The distillery, one of the oldest and most picturesque in the Highlands, passed through several hands until it was bought by Chivas Brothers, a Seagram subsidiary, in 1950.
Used within the Chivas blended whiskies

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Source www.bbr.com :

Strathisla Distillery, Speyside

Strathisla is the oldest running whisky distillery in the Highlands, and has changed hands and names several times in its two centuries of existence since 1786.

Once known as Milltown or Milton distillery, it obtained its current name when it was acquired by Chivas in 1951. As a result, most of Strathisla’s malt now goes into various Chivas Regal blends, and Strathisla single malts are barely promoted at all.

That said, there are several single malt products and their quality tends to be very well-received.