Presuming my 1956 dating for my 4181-3 (SN 004365) slide rule is correct, the 1956 price list has the 10 inch ivorite 4181-3 model for $15.00.

https://www.mccoys-kecatalogs.com/KECatalogs/1954/1956Price/1956kecatprice43.htm

see: https://hypothes.is/a/SmzszMBcEe6Ug29KK3b93A

My 4181-3 slide rule was likely manufactured in 1956 as it has the SRT scale initiated in 1955 and has a serial number 004365 which is in the series 3 segment which reset in 1956 and ran from 0 to 88,500 that year.

https://www.mccoys-kecatalogs.com/keserialnumbers/Dating-2.htm