The shimapan on display are not actually of co-star Mikoto Misaka, but rather one of her clones. However, since they are physically identical in appearance, we can safely imagine this to be what a view of Mikoto wearing shimapan would look like.
We are actually shown a view of what lies under Mikoto’s skirt, but it turns out to be somewhat of a letdown for those without a fetish for boxers.
Titular character Index and her miko-garbed acquaintance Himegami are also shown this episode, but unfortunately their more modest attires of a nun’s habit and miko garb preclude any chance of additional panchira.
Disappointing perhaps, but readers lustful for more pantsu can at least indulge in this animated gif of Mikoto (click thumbnail to view):