On page 2 of Insights it is stated that "once he accepts upon himself not to serve Avodah Zarah, he must observe Shabbos" Would the goy not have to undertake the full rites of conversion, such as milah, before he can keep Shabbos since it is an "Os baini u'vain bnei Yisroel"?
According to this understanding of Rashi (as written in the Insights), the Nochri would not have to undertake a full conversion in order to observe Shabbos. When the Torah calls Shabbos a special sign between Hash-m and the Jewish people, it is referring to the reward for the Mitzvah of Shabbos, as the Gemara in Beitzah (16a) explicitly says with regard to that verse.
M. Kornfeld