According to HMV, the DVD & Blu-ray volume 7 of Ookiku Furikabutte ~Natsu no Taikai Hen~ will include an unaired episode 12.5. Director Mizushima posted in his blog that the OVA was made because they couldn’t put the original manga episodes happened between the episode 12 and 13 within the TV series.
Sources: HMV, Mizushima’s blog
