TrxR,
The 54 round action by itself remained basically the same for over 50 years now.
In the triggers department, there has been some improvements over the years: as far as I'm concerned, the 5071 triggers found on the 1600 serie and the 5018 that equiped the 1800 and 1900 series are far superior to what was available on earlier models.
The barrels were also offered in different lengths and diameters. Some say that the ones manufactured in the early '70 were made of an excellent steel and offered a superior accuracy: quite honesty, even if the rifles made in this era were indeed excellent, I have never been able to see a significant difference in accuracy between these and the newest models.
For benchrest use, unless weight is a critical matter for you (if, for instance, you have to make a certain weight for a given class) I'd go for a model with a heavier barrel such as the 1613, 1813 or 1913.
Hope it helps,
Paul