I just received my corbin seat for the M90 and I didn't order the backrest and they still punched a hole in the leather and put some tacky velcro flap over it. I feel it ruins the clean line of the seat and will allow water to seep in when it rains. FYI when you order for the 90 be specific about whether or not you want the backrest flap cut into the seat.
Mass produced product is the answer. Needs to adapt to whatever option someone may want. IMHO, Corbin needs to make it work for the customer and not their production line. I agree the flap looks like chit if you don't have or want a backrest. I own the complete Corbin package for seating on my 9. I have the front/rear and rear backrest. While I'm happy with th Corbin seats. I do think that their customer service has a lot to be desired and their pricing on new products is outrageous.