|The shirt collars on a First World War Officer were pinned together under his tie. |
Both these Officers in the Royal Engineers wear the same style tunic and cap.
The Officer on the left is from the 1914-18 war, the one on the right dates from the 1939-45 war.
In any photograph but especially in a close portrait such as these, this is the one tell-tale feature.
The Royal cypher on the WW1 capbadge will be G V R
The Royal cypher on the WW2 capbadge will be G VI R