Rummaging around in a dusty old pile of stuff I found a booklet published by the Southdown Enthusiasts Club dating from March 1980. Whilst it doesn’t list outstations as such, the following locations are listed as depots, complete with rail-style two-letter depot codes:
BN - Edward Street, Freshfield Road, Moulsecoomb
BR - Bognor
CB - Crowborough
CH - Chichester
CS - Conway Street
EB - Eastbourne
HD - Henfield
HH - Haywards Heath
HK - Hassocks
HL - Hailsham
HS - Horsham
LS - Lewes
MH - Midhurst
PM - Winston Churchill Avenue, Emsworth, Hambledon, Hayling Island, Hilsea, Hoeford, Leigh Park, Petersfield
SF - Seaford
SG - Steyning
SN - Storrington
UD - Uckfield
WG - Worthing
WK - Whitehawk