Possibly to emboss the amount on a check to make it harder to replace.
Google "antique check protector"
http://www.officemuseum.com/check_p...y_simple.htmThe devices used to grill stamps had a roller with lots of little rectangles of grilled areas (like a little part of that check protector). Whole sheets (of multiple thicknesses, sometimes) would be grilled.
The first grill was an "all over" grill that would put a pattern on the stamp similar to the thing you have. Unfortunately, this made the stamp too weak to separate at the perforations. They then machined out some of the patterns to make them into rectangles. Various sizes and patterns would be used for the next few years.
(edited to add link to antique check protector page)