Scott,
On the Sapporo website which talks about the 14/15 July Air Raids here:
https://www.city.sapporo.jp.e.ain.hp..._21/index.html
I looked at link 11 (on the right side) which was an attack on Shiraishi railway station:
https://www.city.sapporo.jp.e.ain.hp..._11/index.html
The wording is interesting as talks about locomotives “shot by machine guns”. This sounds like a low level staffing attack (probably by navy carrier planes). This might imply that the 14/15 July attack on Sapporo was just the navy attack on those dates mentioned earlier. No B-29s involved.
Unfortunately, I then found this map (click it to enlarge) which shows where the various navy units (planes and ships) attacked at the end of the war. Note there are no navy attacks on Sapporo shown on any dates.
https://history.army.mil/books/wwii/...ages/p_127.jpg