I have stumbled across the acronyms SO and RSO in Kellys Directory. I have tried to find out what the acronyms stand for but cannot locate a 'key' in Kellys. However, wherever I see these acronyms they seem to tie in with records for beer retailers, victuallers or inn keepers. I think that in this context the acronyms may stand for sales outlet or retail sales outlet. If the death record you mention is for a beer retailer then this could be an alternative possibility to those that you have had to date.
All the best
KPRO
