Good morning,
The RGR 1st, 2nd, 3rd and 4th battalions were sent to South Africa in 1904 and returned in 1905. They were disbanded in 1905/6 and the regt disbanded in 1908. This I am assuming was the regtl depot at Fort Widley where records were held until 1905 when they were transferred to Winchester.
5th battalion were sent to Nova Scotia in 1905 and returned dec 1905 and disbanded.
Prior to SA the 4 battalions were in Malta to relieve regular troops for SA duties. Whilst there they wore temperate uniforms, ie; blue serge. The uniform shown would only have been worn in SA.
So that leaves an 18mth window for this photo, May 1904 to Dec 1905. But troops whose time was up would have returned earlier than that date.
John915