In a recent study, de Lange and Glänzel introduced a model for the bibliometric analysis of the extent of multinational co-authorship links. They showed that this model can be considered a generalisation of the "fractionation approach" by Nederhof and Moed. The authors analysed international collaboration links (the Multilateral Collaboration Index) as a function of the share of internationally co-authored papers. The measurement of the deviation of individual countries from (sub-)field peculiarities proved, however, complicated. The intensifying international collaboration and, in several fields, the substantial growth of number of multinational papers (involving three or more countries) in the 90s necessitates a detailed analysis of co-publication distributions, that is, of the distributions of partner countries in a given country"s publication output. The main objective of the study is to elaborate such a measure to be used in addition to the share of international publications and the Multilateral Collaboration Index. In addition, a detailed analysis of national citation impact of domestic, bilateral and multilateral papers in the major science fields is conducted. The model, we develop and the statistical analysis that it allows, support the practical conclusion that the ratio of the number of international links and international papers turns out to be roughly proportional to the ratio of full and fractional publication counts.