Upvote:4
Searching the British Gazette archive from 01/01/1880 through 31/12/1905 for military records of both McLennan and MacLennan with first name Alexander or Duncan (or initial A. and/or D.) yields sparse results:
Commissioning of Alexander MacLennan, Gent., as Second Lieutenant in the 1st Lanarkshire Volunteer Rifle Corps effective 11th May, 1881
Captain A. P. McLennan, 1st Volunteer Battalion, the Royal Welsh Fusiliers resigns his commission dated 4th February, 1899
An unissued Transport Medal as of Dec. 22, 1905, in both the South Africa (1899-1902) and China (1900) campaigns, for a Chief Engineer A. McLennan serving aboard the India.
Private D. McLennan of the 92nd Highlanders received a slight gun-shot abdomen wound 13th December 1879 at or near Kabul, Afghanistan
Note that Admiral Algernon McLennan appears in the Gazette under both spellings, at that rank, so don't be too picky in searches. Clerks were transcribing hand written notes which themselves might not always be perfectly accurate.
Upvote:4
A quick sum up of my guessings:
He's clearly wearing mounting boots and a sam browne style belt, so you might have to look trough officers or cavalry units, or maybe rifles.
I tried to identify the cuffs with some unit, as they are damn singular ( or so i think ). Brittish army cuffs at that time (1900-1914) don't usually show a striked or zig-zagged straight pattern without buttons, and the detail hints at either a crown, a rifles knot, or a dragoon knot. I was unable to find a specific unit related to that "color" pattern, but we know that a single line, althought highly decorated, means 2nd liutenant.
The badge on the collar slightly resembles the Rifles (Scottish and Irish) crown badges, so i looked trough and found the 1rst Lanarkshire volunter rifle corps, with some pictures (older) with more detailed knots from the 1850's and 60's, that strike some similarity.
Pieter's answer has one "hit" on the same unit and rank, so i guess we're talking about 2nd Lieutenant Alexander McLennan. I am unsure whether it could also be a young, 18yo enlisted Duncan McLennan back in 1879 in Kabul, given the name match, but it could make sense.
That would set up the next timeline merging what we know:
Do you have any way of asking for the military record on the current unit's headquarters? This unit merged across the XXth century to end up being part of the Current Royal Scots, and they usually keep account of their officers.