Question

The Rashash, in Sha'ar HaSfeikot ch. 1, raises a difficulty from the fact that Rava follows the position of the Mishnah that a foundling is permitted to marry a mamzeret, but in Yevamot 37a Rava seems to prohibit a foundling from marrying a mamzeret. Nor does it seem from the passage in Kiddushin 73a that Rava specifically explains the Mishnah as saying that a foundling is permitted to marry a mamzeret. We would appreciate an explanation. Thank you very much, Rabbi!