(a) That is an excellent question. (The only one I found so far who points it out is YALKUT HA'MEIRI ad loc.) In fact, I found that the RASHBAM himself (171b) gives another example for Mukdam later in the Maseches. He writes that the Shtar was written on 1 Iyar and predated to 1 Nisan - to avoid this issue.

What did the Rashbam mean then in our Gemara?

(b) I think the answer lies in the continuation of the Rashbam here. RASHBAM points out that the Shi'abud does not always start from the point at which the Shtar is written. If the loan was given before witnesses, and it was specified that there would be a Shi'abud, the Shi'abud begins immediately. If a Shtar is written later on, it may bear the date of the loan in such cases.

If so, when the Rashbam says that the Shtar was written on 15 Nisan, he does not literally mean that it was written then. Rather, he means that the Shi'abud began then - because a loan was given then before witnesses. (The loan could have even been Shaveh Kesef that was given a certain value, avoiding any problems of Muktzah. See Beitzah 28a for examples of loans that were created on Yom Tov.) It was to emphasize this point that he chose that date.

(c) This might answer the question of the RASHASH here. The Rashash asks why Rashbam gives a different example for Shtar Me'uchar: he discusses a Shtar for a loan in which the Shi'abud was created on 1 Nisan, but it was only recorded in writing on the following 1 Tishrei . Why doesn't he give a case of a loan on 1 Nisan that was recorded in writing on 15 Nisan?

The answer is that the loan cannot be recorded in writing on 15 Nisan, since it is Yom Tov! Since he had to push off the date, he pushed it off to 1 Tishrei (an easily remembered Rosh Chodesh date later than 1 Nisan.)

Best wishes,

Mordecai Kornfeld

Kollel Iyun Hadaf