Opps found my answer
Just leave this here for the archive's so when I forget I can come back.
*Please Note: When sizing Speedfeed stock sets for Remington 20 ga. guns, those with serial numbers ending in "K" or "U" are small frame and require a 20 ga. stock set. 20 ga. guns with serial numbers ending in "X" or "N" are large frame and require a 12 ga. stock set