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There is a small date code in the form “YM” engraved into each slide rule but often not colored. Often it is in the lower left of the rear of the rule but can appear almost anywhere.
I was unable to get an example to show up using a scanner as the markings do not stand out at all.
This cursor may not be authentic Hemmi but the rule it came on is consistent with a date of about 1913. The plain version (non magnifying, non decimal-indicating) appears only in the 1913 Tamaya catalog.
Magnifiying and decimal-indicating versions (but not plain versions) appear in the 1914 Hughes-Owens and the 1919 Tamaya catalogs.
Hemmi was granted Japanese patent 22129 in 1912 for laminated bamboo construction and success seems to have followed quickly thereafter. in Tokyo started selling Hemmi slide rules in 1913 as did the Hughes-Owens company of Canada in 1914.
The Frederick Post Company of Chicago began selling Hemmi slide rules in 1931.