Coat of arms used by the prince-bishops of Sion of the Riedmatten family.
The prince-bishops used their family coats of arms. The Riedmatten coat of arms was in use for much of the early modern period
(1529–1545, 1565–1613, 1640–1701) and is  presented as coat of arms of the bishopric in e.g. Siebmacher (1605). 
La ville de Syon auec les deux chasteaux &amp; l'eglise cathedrale appellée Valeria Basel 1552, (mapandmaps.com)

The sword and crozier shown as supporters in the print by Münster are adopted as heraldic charges in the modern (19th century?) coat of arms of the diocese.
Own work
User:Dbachmann, derivative of File:Wappen Burgdorf (Baddeckenstedt).svg, File:Blason fam fr Bonaparte bis.svg