The /ʊ/ vowel is usually spelled <oo>, <u>, and <oul> in could. The /aʊ/ diphthong is spelled <ou> or <ow>. The computer also found the pairs luring/louring and tour/tower, but I have taken them out since the /ʊə/ diphthong in tour and luring is best treated as a distinct phoneme, and some speakers would use a palatal glide in luring in any case.
This is a contrast between a short vowel and a diphthong, fairly close in their starting positions in the mouth. There is some likelihood of confusion though not associated with any one group of learners.
The density figure is 0.71%. The set makes 6 semantic contrasts giving a loading of 42.9%.
could cowed full foul fuller fouler fullest foulest fullness foulness fully foully fuller fowler fullers fowlers poorhouse powerhouse poorhouses powerhouses put pout puts pouts putsch pouch putsches pouches
John Higgins, Shaftesbury, December 2009