The Officers Model shared serial numbers with the Official Police from 1927 through c. 1949. Wilson's wording is confusing, but the table on P. 354 is for .22 revolvers. He says to refer to other sections for other calibers. 621xxx was made in 1937 as listed under the Official Police Model, and 643xxx in 1940.
It is not a matter of an OP or OM frame; the frames were the same, and the same as the old Army Special.