Sako S491/M591/L691/L691 Magnum are all the same family, and the generation before the Sako 75's.
The S491 is a short action for calibers such as .17 Rem, 5.56x45 etc, M591 is a medium action for calibers such as 7.62x51/.308 Win, .270 etc. L691 is a long action for 6.5x55, .30-06 etc. L691 is a long action for Magnum cartridges.