good news, the sourcecode of app251 and sys150 has been released here:
github.com/Seeed-Studio/DSOQuad_SourceCode
Where is the App251 firmware in object (i.e. usable) form? I see only App250 at this point in time.
UPDATE: I found three APP_B251.hex files in the Exe directory in git (one is 1APP_B251.hex). Tried one of them but it did update properly. So I’m still not sure which is the most up to date firmware file.