one of the oldest written words in the english language still in usage

[Michael Adams]

No, the prepositions aren't merely Kentish -- they're in the 601 quotes (I've checked since I last wrote).  The problem with antedating in the OED is that the OED has to present the word in the first quotation -- this is usually true, even in the case of the functional words, but not always -- sometimes you have to recover the first citations for functional words in quotations for other words, you know, the nouns :-).