These photographs are marked as being the work of Harris Son & Co., Merthyr.
Looking at old newspaper notices and adverts, the firm appears to have traded under the name Harris & Co. from 1871 to December 1877. From 1878 it traded under the name Harris Son & Co.
Robert Dodds Scott (born about 1802, Ancrum) died in 1873.
It would appear that the photo alleged to be RDS was therefore taken sometime after his death.
If anyone knows the identity of the man shown, please let me know. He's almost certainly a Scott from the Merthyr Tydfil area of Glamorgan, Wales.