Tables are presented from which exact values of the Wilcoxon distribution may be obtained when the smaller sample size $m$ does not exceed 12. The Edgeworth approximation to terms of order $1/m^2$ is given and its accuracy investigated.
"Significance Probabilities of the Wilcoxon Test." Ann. Math. Statist. 26 (2) 301 - 312, June, 1955. https://doi.org/10.1214/aoms/1177728547