The Model 20 is an old time revolver built on the large N frame. Original grips for this gun would be wooden service (magna) grips. You might want to consider a newer pair of S&W checkered target grips with a speed loader cut out. They are getting hard to find. Expect to pay top dollar for a pair unless the seller doesn't know what he has or they are in poor condition. An alternative to consider would be installing rubber Hogue grips, combat style with finger grooves. They are cheaper and should provide room for speed loader use. PM me if interested, I should be able to set you up.
rfo1