ready reckoner vs reckoner

ready reckoner and reckoner both are nouns.

Word	Noun	Adjective	Verb	Adverb
ready reckoner	Yes	No	No	No
reckoner	Yes	No	No	No

As nouns, ready reckoner and reckoner are synonyms defined as:

ready reckoner and reckoner: a handbook of tables used to facilitate computation

ready reckoner (noun)	reckoner (noun)
a handbook of tables used to facilitate computation	a handbook of tables used to facilitate computation
	an expert at calculation (or at operating calculating machines)