According to Ludovic Ducrocq (brand ambassador for Glenfiddich) the Straight Malt "was the first triangular bottle of Glenfiddich, which pioneered the single malt whisky category in the early 1960s". So historically probably quite significant.
McTears auction from 2004 has the following listed:
595 The Glenfiddich Straight Malt
Product of The Glenfiddich Distillery, Dufftown. Distilled and bottled by
William Grant & Sons Ltd., Dufftown and Glasgow. 26 2/3 fl. ozs. Minor
scratch to base of label at centre. Screw cap, embossed lead capsule.
Level: 2.5 cm from base of capsule. Single malt, 70 degrees proof
£250 - 300
So with general Whisky price inflation with a bottle in good condition, I wouldn't be suprised if one didn't make over £300!
YMMV - Rufus.