Imports into the states had to have the point of origin plainly stamped, so the pottery stamps can actually date the piece. If a mud figure has an indentation or a black or red stamp with "CHINA" (sometimes the "N" is backwards) or "HONG KONG", then it is from c. 1890 - 1919. Those figurines stamped with "MADE IN CHINA" or "MADE IN HONG KONG" date from c. 1920 - 1944. Occasionally you'll find "MADE IN CHINA" or "CHINA" stamped in red ink, dating from the late 1940s. Those that only show the "CHINA" in red ink from the late 1940s should have been stamped "MADE IN CHINA" and are few in number and may simply be a result of improper stamping (only part of the stamp being placed against the edge). From 1952 on, the mud figures have a stamp and a number.
Occasionally you'll come across a mud man with no markings. Travelers or missionaries to the orient would purchase the mud figures at local markets and carry them home. These were not stamped, because they were not for importation.