Paul Beale, in _A Concise Dictionary of Slang and Unconventional English_, edited from _A Dictionary of Slang and Unconventional English_ by Eric Partridge (London: Routledge, 1989; ISBN 0-415-02807 (hardback--it's also available in paper)), has a quite long article in his Appendix on "HARRY as a meaningless prefix" (p.525). The gist whereof is that he takes it as originally RN usage, starting from a slang expression "to drink at [Henry] Freeman's quay." By some route it got to be attached to other terms, always with the '-ers' stuck on the end, so he quotes a description of the English Channel off Dunkirk in 1940 as Harry flatters. He also notes the extension of the expression from the services to the "young, smart set" in the late '50s. I recollect that at University, when I was wasting my time rowing rather ineffectually about 1963-4, the college First VIII had a call for a spurt of "Harry clappers, chappers" which, to tell the truth, the rest of us thought was a bit OTT.
Speaking of wasting time, Beale's revision of Partridge is a wonderful book to get lost in. In the appendix, he also has a pretty full account of the "Oxford -er," which apparently started at Rugby School.
