There's also an 1960 article on this by Hirayama sensei: 「喫」の北京音の由来: https://t.co/uLv02y7Jrd -- He seems to assume dialect mixture and an early (dialectal) development like khek > *khjiek > *tsyhek (converting his to Baxter's notation). Thus, MC *tsyh- > MSM ch-. Hm. 5/