I too, like rvaloe, suspect that it came up sometime in the late 1960s or 1970s. My mother, who came to Germany sometime in the 1960s, wouldn't dream of leaving out the "past" and definitely taught us "half past". I consciously became aware of the usage without the "past" sometime in the 1980s, I think, but since I was afraid of mixing it up with the German "halb zwölf" I didn't even try using it to provoke my mother, who is a bit snobbish about speaking "proper English"* ;o) It never became part of my active vocabulary.
*My mother's "proper English" is probably spoken by hardly anybody on the British Isles nowadays, since her English evolved much less in her German environment. Even listening to BBC Radio 4 and frequent visits to England in our school holidays and later, to family gatherings etc. have not managed to change it much. No regional accents, glottal stops, dropping aitches etc., no new colloquialisms and most certainly no four-letter words, thank you very much.