Hi there - did you ever find a replacement? I'm doing a bike build just now, but the c40 acb headset was supplied with the frame. I either didn't notice the ms100 part was missing on arrival or have somehow lost it, but after some research I am also starting to wonder if it is a separate part at all. I only thought I might be missing it when I looked at the parts diagram published by FSA available for download here: https://www.fullspeedahead.com/en/support/library/documentation?category=139
the ms100 is on the diagram sitting below the cone spacer; however, I don't ever remember seeing it. When I run my finger on the underside of the cone spacer I can feel a ridge of rubber running around the circumference just on the inside. Leads me to the conclusion that the ms100 is factory bonded to the cone spacer in the same way that the part labelled on the diagram as MS053(872) is (the rubber washer at the top of the cone spacer). Certainly when you look online any retailer that has an image of the box contents of the headset does not include a part similar in appearance to the MS100 eg
So I'm planning on completing the build with the parts I have - your post is one of the few online references to the the ms100 part so wondering if at this stage you reached the same conclusion or know any different?
i did get to source a new seal for the fsa but it had a 2 month lead time, what you say about it being bonded might make sense although when i took the headset out prior to a respray it was fine and with the rebuild i couldnt get the steerer tension right as it was either too loose or to tight with no inbetween
Thanks for this thread. I was given an adapter to convert the my crown race into a 1 1/8" from 1 1/2". The part has a tapered bevel and no joke looks identical to the rubber seal ms-100. It has the same bevels and all, so I was a little confused. It all makes sense now and I can start my bike building finally lol. Thanks for the information guys!