New Shamisen player in Vietnam, have some questions about the Sawari

Hello Bachido, greetings from Vietnam!

I recently bought this shamisen for the equivalent of $25 from a very very old man who teaches mandolin in my neighborhood (he listed the thing on FB Marketplace interestingly enough). Seems to be a karin, mitsuori-sao Nagauta (based on sao size).There’s no strings, bachi and koma (though he did give me a sanshin koma as a gift?) but the rest of the shamisen seems intact.

It seems a lot of the shamisen I saw has the “azuma-sawari” mechanism installed where they routed a hole through the sao after the kamigoma. This particular shamisen has the sawari installed right in the tenjin. My questions are, what is this type of sawari called? Is there any history behind it? Have any of you saw anything similar?

Anyways, appreciates any answer, couldn’t wait til I have all my essentials to start playing.

Another angle of the sawari: