Most Frederick Post slide rules from 1925-30 are marked "Pat'd. U.S.A. 1/31/22."  This refers to US Patent 1,405,333 issued to Herman Ritow of Chicago for 50- or 100-cm-long scales cut into 25cm segments which are placed side by side on a linear slide rule.   Thus a convenient length "10-inch" slide rule can hold a 100-cm-long scale.  Precision and accuracy are considerably increased but calculations become laborious when, for example, you have figure out which of the four segments of one scale you should be matching with which of the two segments of another scale.  The patent information is available online from the US Patent Office and there is considerable information in Ritow's book: "Elementary Slide Rule Manual"  published by The Frederick Post Company, 3635 North Hamlin Ave, Chicago, ILL.  Copyright 1925, 86 pages.  (The first 52 pages are a conventional slide rule instruction manual but the last 34 pages cover the theory and use of "Ritow Type" slide rules.)

"Pat'd. U.S.A. 1/31/22" appears on nearly all Frederick Post and Post Ritow slide rules from 1925-30.  The few rules which don't carry the patent date are from early in the period when Koch, Huxhold & Hannemann was still using the same paper table ("Slide Rule Data Slips") that they had been using for other English production; it didn't carry the Ritow patent date and didn't mention the Frederick Post Co.  However, KHH soon switched to another paper table, "Table of Constants" which did carry the 1/31/22 date and named The Frederick Post Company.

Although  "Pat'd. U.S.A. 1/31/22" appears on all except the earliest Frederick Post slide rules from 1925-30, the patent applies only to "Ritow" rules.  The vast majority of Post slide rules from this period are conventional designs to which the patent does not apply.