Chemistry Slide Rule
Frederick Post model 1491 [1960-c1969]

Hemmi model 257.
Hemmi  model 257 was made from 1952 until at least 1968.  Sometime between 1968 and 1971 the scales were changed and the model number became "257L."  All the Frederick Post Chemistry Rules I have seen are Hemmi models 257--not 257L.

This slide rule first appeared in the 1960 General Catalog but date codes as early as 1956 are known.  It last appeared in the1966 Catalog but was probably available for a few years after that.
1960 Frederick Post General Catalog,  Pages 82-83

Date code "HI"  (September 1957)