The attempt to link Japanese and Korean by Alexander Francis-Ratte seems too bold to some linguists, but I think it’s too timid in the specific changes he acknowledges. For OJ yana ‘fishweir’, Ryu. yama, the change of m > n seems clear (just as in \*myawko > Ainu meko, OJ nekwo ‘cat’ and \*(ka)myira ‘garlic’ > OJ myira, J nira), but for some reason he assumes a new compound ‘fishwife’, which makes no sense. Further, though he sees a related compound in J take-yarai ‘fishing trap made of bamboo’, he doesn’t believe n > r either. Considering the possibility of n > y in \*wani ‘saltwater crocodile’, \*wani-samba > \*wayi-saba > \*wai-saba > Middle Okinawan waisaba, it seems both internal and external evidence shows many alternations involving n. This would make his other evidence of r corresponding to y, etc., suggest, for example, \*pwor > \*pwoy > OJ pwi ‘fire’, but \*pwon- > pwo- (with nasalization) in compounds, which is very similar to Indo-European \*puhor / \*puhon-, also with final -r but -n- in other environments.
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In addition, the change of p > m in \*pwoy ‘fire’, mwoya- ‘burn’ ( > moe-) is the same seen in another supposed Indo-European borrowing: \*kaput- ‘head’ > \*kam(p)uto > OJ kabuto ‘helmet’, MJ kabu(ri) ‘head’, Nase kàmàčí (and maybe OJ kamyi ‘top’). The supposed loanword being an item of armor, which could be borrowed for cultural reasons, also makes it more likely that a connection exists for Greek mákhaira ‘knife, sword’, J masakari ‘broad-axe’ (with m / p possibly seen if related to Middle Korean pskúl, Kor. kkeul ‘chisel’). All these m’s remind me of supposed Indo-European \*medhu ‘honey’ > Middle Chinese myit (possibly also the source of J mitsu). Since the change of e > ye (or yë or Yï or a similar sequence) is reconstructed for Tocharian, which is the Indo-European language that moved furthest east in ancient times (later splitting into two, A and B), and closest to China, it would be the simplest explanation for some of these loans, with older \*myïtï becoming both TB mit, MCh myit.
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The presence of m in all these words makes the similarity of m- found in Tocharian look interesting:
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TB mit, MCh myit, J mitsu ‘honey’
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TA mkälto ‘young’, OJ myidu- / myitu-myitu-
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TA malto ‘in the first place’, OJ madu, MK mwoncye ‘at first’
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TB mewiyo ‘tiger’, \*myawko > Ainu meko, OJ nekwo ‘cat’
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Seeing -u > -u and -o > -o in 1 each and 2 examples of final -to > -du in OJ is compelling, as is -lt- becoming either t or d (myidu- / myitu-myitu-), similar to reconstructed -rk- in Kor. to k or g in OJ (if MK ùsïrk ‘badger’ was related to OJ usakyi (East OJ wosagyi) ‘hare’). Also, the optional -wo- or -o- in mwoncye was explained as optional rounding of older reduced vowels next to m-, just as in \*mëlto:(n) > TA malto (quite a coincidence, if unrelated). Since there’s no reason why only words beginning with m- would be borrowed (5 if including masakari), it’s possible there were many more loanwords that changed beyond easy recognition (simple enough if changes like p to m existed). Other loans might have been names found in myths, like OJ woroti ‘big snake’, very similar to Avestan vërëthra- from \*wërëtra-, obviously the same name and myth as Vritra. These have never been gathered together and evaluated as a group, so the nature of the type and direction of borrowing or common origin remains unclear.
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OJ (Old Japanese); MJ (Middle Japanese); J (Japanese); Yon. (Yonaguni); Ryu. (Ryukyuan);
MK (Middle Korean); Kor. (Korean)
MCh (Middle Chinese); Ch.