Side-ring EXTRA · Designation of the pair
Yoma 41a (Rav Chisda), registered by Torah Temimah on the parallel zav verse 15:15.
English: Rav Chisda said: Nest-pairs are designated only either at the owners’ taking or at the kohen’s performance.
When chatas/olah roles are fixed
The same rule of designation that applies to the zav’s birds applies to the zavah’s: the pair’s roles may be fixed when she takes them or when the kohen offers them.