Predictors of early survival in Soay sheep: cohort-, maternal- and individual-level variation
Owen R Jones,* Michael J Crawley, Jill G Pilkington, and Josephine M Pemberton, Proc Biol Sci. 2005 December 22; 272(1581): 2619–2625. Copyright © 2005 The Royal Society
Their data set was the 1989 to 2002 surveys of Soay sheep on the island of Hirta in the St. Kildas, so it is "real" Soay sheep.
While not explicitly publishing lamb weights, they reported them in their analysis on survival factors. From their Figure 2a, shown here:
I extracted their data from the figure above and calculated a mean value for weights in pounds.