Is there any phonetic rule behind readings 馬場{ばば} and 馬場{ばんば}?


I understand that place names often undergo changes separate from regular
language, but I always wondered if there was any phonetic rule behind reading
for the word 馬場. Meaning "horse riding grounds" and being a designated area in
most castle towns, it converted into contemporary place names, yet in some
areas it is pronounced mostly ばば, but in some places ばんば. Most notably:

  * 東京 高田馬場{たかだのばば}
  * 大阪 馬場町{ばんばちょう}

This [wiki page](https://ja.wikipedia.org/wiki/%E9%A6%AC%E5%A0%B4%E7%94%BA)
shows ばんば only in Ishikawa, Aichi and Osaka Pref. However having visited a few
castles I remember seeing ばんば elsewhere (e.g. 桜の馬場{ばんば} in Kumamoto).

1) Is there a more general phonetic rule behind this? Are there other examples
of words in which repeated syllable gets an ん before?

2) Could such rule (if existed) be tied to a regional dialect (like ん is
placed where や is used in areas where や replaces だ ...though I doubt)?

Or is it just purely customary?




Section 1 of the appendix on Japanese phonology in NHK 日本語発音アクセント辞典 suggests
that pre-nasalisation of /b/ is found in dialects in what it describes as
東部方言, covering all of Japan east of 石川県 and north of 愛知県, precisely where you
observed that the wiki page has ばんば. This should indicate that there's an
isogloss somewhere in the region.

I have no information on why Osaka or Kumamoto would have ばんば place names
though: the book indicates that pre-nasalisation only occurs in Western
dialects (西部方言) before /g/ and /d/. The book does not describe pre-
nasalisation occurring in any Kyushu dialects.

