I ordered AR31C/ACE on 8 April 2022. The model Thoman sent me had some features of the AR81C/ACE (item n. 491475): a darker colour, machine heads with chrome buttons, 21 frets (the AR31C has 20).
There are no details about the batteries of the BR-2.1P, so I contacted Baton Rouge. They wrote the BR 2.1P system uses 3 x CR 2025 coin cells and confirmed the length of the Auditorium (OOO) AR31C/ACE is 104 cm. This means it fits in a Harley Benton LightCase-Dreadnought (item n. 511119). I pad this case with microfiber cloths from my local supermarket.
Fitting a guitar strap can be a problem. I got an Levys Cotton Acoustic Strap 2" BR (item n. 472539). The AR31C pickup output at the bottom doesn't fit the strap's end piece, so I fitted a MusicNomad MN270 Acousti-Lok strap lock adaptor (item n. 470773). That feels secure, but to be safe, I placed a lock and lock wheel from Schaller S-Locks M CH (item n. 438346) on the end piece. The lock snaps on the the MusicNomad strap lock adaptor.
Overall, there are no issues: no fret sprout, frets line up, fretboard is straight (no twists or warps). The BR-2.1P is quite a gem and works well. The AR31C/ACE competes well against other guitars at this price (€361 on 31 July 2023).