I compared the live weight of lambs with the meat weight - i.e. the carcass processed by the butcher. The carcass includes just the bone and muscle structure, not the internal organs, skin or contents of the body cavity.
Here is the result:
|Date||Live Weight at Slaughter (kg)||Meat weight (kg)||% Meat /Live||Bottle Fed|
The ratio of meat to live weight seems - this is a very, very small sample - to be higher in the lambs that fed naturally. Naturally feeding lambs had an average 51.4% meat:live weight ratio, while bottle fed lambs had an average 42.8%.
There are some published papers on this, for example this paper on milk source and body weight  but none, that I know of, on Ripollesa sheep.
Time for more research, methinks!
Joaquim Casellas at the Autonomous University of Barcelona has confirmed that there are no studies of this issue in Ripollesas. He has pointed me to this useful study on goats which shows that maternal milk vs milk powder does cause differences in fatty acid composition.
This leads me to think that one possible explanation of the differences in meat weight:live weight is that bottle fed lambs may build up more body cavity fat (which is discarded in the butchering process) than ewe-fed.
1 Hernández-Castellano, L. E., I. Moreno-Indias, A. Morales-delaNuez, D. Sánchez-Macías, A. Torres, J. Capote, A. Argüello, and N. Castro. ‘The Effect of Milk Source on Body Weight and Immune Status of Lambs’. Livestock Science 175 (May 2015): 70–76. doi:10.1016/j.livsci.2015.02.011.