The 28H hub limits your choices on rims, my hub is a 36H. Here's a couple of Sun rims from bikepartsusa, it says they are M-13 rims, 20 x 1-1/8, sounds like 451 rims to me. Perhaps Raleigh put 28H hubs and rims on the 451 bikes and 36H hubs and rims on the 406 bikes?

http://www.bikepartsusa.com/product_...ml?p=01-124371
http://www.bikepartsusa.com/product_...ml?p=01-124369

20-inch rim page:
http://www.bikepartsusa.com/view.pht...m&f_c2=20+inch

Wheel sizing chart:
http://www.bikecult.com/works/wheelsizes.html