[NTG-context] Getting ONE, TWO... for chapters

Am typesetting a book that has 'STUDY ONE', 'STUDY TWO' etc. in place of CHAPTER ONE, etc. I have no difficulty getting 'STUDY' with \setuplabeltext[chapter=STUDY~], and I can achieve 'STUDY One' with the key conversion=Words in \setuphead[chapter]. My question: how do I get ONE instead of One. I tried conversion=WORDS (or WORD), but that is not recognised and the output then becomes STUDY 1, which I don't want. I also tried playing with \definestructureconversionset[myconversion][ ,A,] then adding the numberconversionset=myconversion to \setuphead. There must be a simple way to achieve the desire result, and to be honest I'm wondering why WORD does not work? Any thoughts? Julian