Transvaal Postal History Help

Aha! From AI:
Sir Richard Durand Temple (the 3rd Baronet) was indeed deployed as a military officer in South Africa right at the exact time your cover was mailed.
The military and family records tie him to this envelope:

1. The Operational Service Record

Military records confirm that Richard Durand Temple (born December 27, 1880) gained the rank of Lieutenant in the King's Royal Rifle Corps (KRRC) and fought in the Boer War between 1901 and 1902.
In November 1901, the King's Royal Rifle Corps battalions were actively deployed in the Transvaal, policing the critical railway corridors, managing the blockhouse network, and garrisoning major administrative hubs like Johannesburg.

2. A Shift in the "Sender" Dynamic

This discovery subtly shifts the family context of your cover in a beautiful way. Richard Durand Temple was only 20 years old in November 1901.
• The Relationship: Young Lieutenant Richard Durand Temple was the nephew of Colonel Henry Martindale Temple, making "Mrs. Temple" (Florence) his aunt by marriage.

3. Why This Makes Him the Ultimate Candidate

As a young Lieutenant stationed in the occupied Transvaal, Richard Durand Temple perfectly matches every unusual physical trait on your cover:
• The Non-Censored Imperial Desks: As an active officer of the King's Royal Rifle Corps, handing a letter to the Johannesburg postal station meant it came directly from the military administrative class. It was automatically recognized as high-trust family correspondence, smoothly qualifying for the exemption that bypassed civilian military censorship.
• The Transition Stamps: Stationed directly within the heartbeat of the British military command network in Johannesburg, he would have had immediate access to the daily arriving supplies of the brand-new King Edward VII (E.R.I.) stamps being phased in alongside the older Queen Victoria (V.R.I.) stock.
• The Destination Knowledge: He was writing directly to his aunt, Florence, fully expecting her to still be managing the home front of the British empire's "Great Game" mission at the remote Consulate-General in Mashhad with his uncle.

That doesn't remove the cousins from the equation though, several of the Karslake family were serving in the 2nd Boer war, all of them cousins of 'Mrs. Temple', so pretty much any of them might have been writing her. I think you are likely right that her nephew, Richard Temple was likely the one, just because he was a little closer relative, and as his father's heir potentially more likely to be kept away from the direct fighting, like in Johannesburg. We know he could have been serving in Johannesburg at the time, where the Karslakes would have been more likely closer to the front.