Bill... a little more intel.
From 1906 till 1922 Marlin produced six different models of 22 rim-fire pump rifles, all look similar to yours, the model 18 (1906-1909), 20 (1907-1920), 20 S (1920-1922), 25 (1909-1910), 29 (1913-1916) and 32 (1914-15). Most models have their slight differences. From what I have found the only reference to a 20” barrel is the "Model 18 Baby Slide Action". It came in either round or octagon 20”barrel. Plain straight grip stock and slide handle (certain other models had a grooved slide handle).
It appears 1906 was the first year Marlin started building firearms. If yours is a model 18 it was their first attempt at rim-fire pump rifles, maybe their first rifle of any type.