1) Is the Rambam with or without a double knot at the top? 2) Is it true that we have archaeological evidence that the halacha for the number of strings follows Tosafos?
1) The Rambam himself does not speak of any double knot – nor does anyone from his time or earlier. The double knot was introduced by the Tosafot. They reasoned that a knot should be a knot that is forbidden on Shabbat. However this is not the opinion of other poskim. When the Gemara talked of a knot for tzitzit – according to Rambam, Rashi, Raavad, Rav Amram Gaon, and others – the intent was a knot made by wrapping the shamash around the core strings.
Now, since the “kesher elyon” (the “upper” knot) is a biblical obligation many are stringent and use a double knot, thus fulfilling the most stringent opinion on the matter (that is, Tosafot would not consider the single wrap a “knot”, but Rambam would agree that a double knot is a valid, though unnecessary, knot).
There is a mahloket (legal dispute) as to whether the “upper knot” is the one first tied closest to the garment edge, or the last one tied at the bottom of the wraps. Due to this, some are strict and make a double knot at both the top and bottom of all the windings. That being said, one who wants to follow the Rambam’s words precisely makes no double knots whatsoever.
2) I am not aware of any archeological evidence supporting any method of tying. To the best of my knowledge no one has ever found a talit tied with tzitzit (this owing in part to the bio-degradability of organic fabrics). I have heard that a pair of tefillin according to R’Tam has been found in an archeological dig.
– Mois Navon.