I also had to replace my 121 M1 stock; however, finding one proved difficult. Moreover, there were conflicting stories whether M1S90 stock would fit the 121. In fact, I email Benelli Italy with this same question, but my message went answered. Perhaps if I had written it in Italian, I may very well could have gotten a response.
In the end I settled for a set of original wood furniture that I found in Italy. If you are still in need of one, drop me a line.
While not the original, CDNN has SL80 stocks, which will work with 121 M1; however, the forearm from an SL80 will not fit on the 121
Good luck,