To my surprise I find the OED cites less examples than I'd expected. Normally these fictitious rules were routinely broken by people like Dickens, but there are no examples for several centuries before his period.
The first use of less in sense 1.c., as a synonym of fewer, is from King Alfred's translation of Boėthius:
Swa mid lęs worda swa mid ma
where it's originally an adverb governing the partitive genitive worda.
Then Caxton wrote By cause he had so grete plente of men of hys owne countre, he called the fewer and lasse to counseyll of the noble men of the Cyte.
Then Lyly wrote in Euphues that I thinke there are few Vniuersities that have lesse faultes than Oxford, many that have more.
(And no I'm not putting forward Lyly as an arbiter of elegance.)
Then there are only modern quotations, second half of last century onward; and it looks as if it first got into print in mathematical writing, where naturally you don't want to pronounce the relation in a < b differently depending on what the entities being compared are.
The OED says the synonymy with fewer is 'Freq. found but generally regarded as incorrect'.
