The following is from 1961 manual:
«The reason for the left and right specific speakers is that the side mounted drivers in this TriAxial surround are mounted out of phase with each other, is to create a large, rear surround field. The side driver that is in phase with the front mounted drivers, needs to be the one pointing at the front speakers for optimal surround effect placement and steering. This means that the TriAxial surrounds are mirror image pairs.»
I have contacted Arendal support to confirm whether the 1961 surround are left and right specific as the manual says or if they are identical.
Will update the article once I get a confirmation.