I’ve been looking a bit into Medhursts old dictionary, only to realize that although my Taiwanese tends to Chiang-chiu like pronunciation, I have only a very limited knowledge about the sounds in actual Chiang-chiu.
The 15 initials posed no problem, this much I know at least and after all, Medhurst’s transcription doesn’t differ much from later POJ in that point

Final no. 22 兼 “-ëem/-ëep” clearly stands for POJ “iam,” however both the transcription and Medhurst’s description of the sound indicate that the final sounded rather like [iem] (I’m guessing similar to the way POJ “ian/iat” sound, but with a bilabial in the end. I have never heard of this phenomenon, neither in discussions about the peculiarities of Chiang-chiu variants nor in any account of other variants, ancient or modern. Can anyone confirm that this final is indeed pronounced with an [e] or [ɛ] sound in Chiang-chiu? Or did the sound change over time maybe and is now “iam”?
Final no. 5 嘉 “ay” definitely stands for the open [ɛ] knew existed in Chiang-chiu (rhyming with 假、嫁、下). However, Medhurst lists another final, 伽 (no. 39) which he transcribes in the same way and describes as very similar to 嘉 but slightly distinct (it is of course distinct from POJ “e” (稽 “ey”) as well). 嘉 he describes as the vowel in “care, bear, wear ect.” Since Medhurst was an Englishman, this translates to [ɛ] in my opinion. 伽 he describes as the vowel in “fate, gray, may.” From the perspective of contemporary English, I would transcribe this as [ei, ɛi] or possibly [ɑi], which sounds very different from [ɛ] to me, so I’m wondering whether Medhurst’s mid-19th-century English had a different sound value for these words. The Si̍p-ngó͘ Im lists as rhymes for 伽 only three characters: 莢, 瘸 and a third character which I can’t decipher. 伽 I can only find as “ka,” 莢 I find as “ngeh” (“ngoeh” in Choân-chiu) and 瘸 as “khôe” (“khê” in Choân-chiu). Douglas lists “ngɛh, a pod of grass” (being 莢), but nothing with “ɛ” which would fit 伽 or 瘸. So from todays perspective this final seems to have split and merged with at least two other finals, Chiang-chiu “-ɛ”/Choân-chiu “-oe” and Chiang-chiu “-oe”/Choân-chiu “-e.” Can anybody confirm that?
The situation with the o’s also leaves me a little confused. I seem to remember to once have heard from somebody that traditional Chiang-chiu doesn’t have the open “o͘” (the one with the dot), but I wasn’t sure about that, so I approached the matter with no real expectation as to the existence of this vowel. Indeed, the Si̍p-ngó͘ Im (and therefore Medhurst) distinguish two kinds of o’s: 沽 “oe” (no. 11) and 高 “o” (no. 15). The former rhymes with syllables like 古、湖 and so on, which I would all pronounce with the open “o͘,” while the latter corresponds to the closed one. Medhurst describes the vowel of 沽 as rhyming with English “toe” but “with a full mouth.” The vowel in “toe” (again from contemporary English perspective) is a diphthong to me, something like [oʊ], but a different way of pronouncing the same phoneme doesn’t seem all that odd to me. However, the list includes nasal variations for both of the two o’s: 姑 “ⁿoe” (no. 41) and 扛 “ⁿo” (no. 49). My vocabulary still only contains a very limited number of syllables with a nasalized o, most of which have a nasalized initial (such as the literary readings of 五 and 腦; the only one without a nasal initial that I can think of is the sentence final --honnh), but I had always lived under the impression the nasalized o’s in Minnan usually become open by default, so there would only be open nasal o’s anyway. Does that mean that this is a more recent development and the nasalized open and closed o were still distinct at least in early to mid-19th-century Chiang-chiu? What makes the matter even more confusing to me, is the fact that neither 姑 nor 扛 don’t seem to have any kind of nasalized o in other dictionaries I looked them up in (and actually such a vowel would surprise me especially in the case of 姑 which doesn’t have either a nasalized vowel or a nasal initial or final in any variant of Hokkien I know of, nor in Mandarin).
I actually don’t have any concrete questions about these finals (apart from whether or not “iam” is indeed pronounced as [iem] in Chiang-chiu), they have just surprised me, especially 伽 and the two forms of nasalized o left me wondering whether Chiang-chiu and Choan-chiu were really as phonetically close as I thought (which wasn’t actually that close to begin with, but I had always thought it possible to break the differences down to a few basic rules). So, if any of you has more insight to provide, I’m happy to learn
