I'm told that Arthur proposed to Florence by letter from France during WWII (causing her famously to start blushing and run from the diner table).

Arthur wearing the uniform of the South African Infantry (click to enlarge).
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We can see on 2 Oct 1908, Arthur, his mother and four of his siblings sailed from London to the port of East London, South Africa aboard the ship GALEKA of The Union Castle Mail Steamship Company. They travelled 2nd Class:
Mrs E Knibbs 45
Mstr A R Knibbs 16
Mstr H Knibbs 13
Miss M Knibbs 7
Mstr N Knibbs 5
Miss I Knibbs 21

I'm assuming this was returning home to South Africa following a visit to family in England.
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We're told by Owen Knibbs that Arthur Reginald Knibbs was wounded in the Battle of Delville Wood which took place between July and September, 1916, during WWI. After being wounded, he completed his wartime service in England censoring soldier's correspondence. He went back to South Africa in 1920 after being awarded an MBE.

Delville Wood was sometimes known as Devil's Wood, and the fighting there during the battle of the Somme was particularly ferocious. The majority of the wood was eventually taken by South African soldiers on the 15th of July 1916, and they held on grimly during numerous German counterattacks for six days, until they were relieved.

The London Gazette Supplement, 3 June 1919 has the following entry under South African Forces:
To be Members of the Military Division of the said Most Excellent Order:
Knibbs, Lt. Arthur Reginald, S.A.I.

This relates to the Most Excellent Order of the British Empire (MBE)

See the Notable KNIBBS Page in honour of Arthur Reginald.
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Arthur is listed in the book named "Rollcall - The Story of Delville Wood" written by Mr Ian Uys:

Second Lieutenant A.R. KNIBBS - 1st SAI, D Coy - Wounded on the 18th July - Later awarded the MBE (Military Division) and mentioned in War Office Communiqués

Arthur's brother Horace was killed at Delville Wood.
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In 1951, Arthur Reginald Knibbs could be seen living at 16 Smuts Road, East London, Cape Province, South Africa
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Arthur visited the UK in 1956 and appeared to be travelling with a Mrs Knibbs, age 55, so born about 1901. The document states that she is married and a housewife, but so far as I'm aware, his only wife was Florence who was born in 1895. Certainly, that was the date calculated from when they travelled to England in 1960. I did wonder if perhaps it was his sister Marjorie who we know was born on 1901, but she had been married so wouldn't have been Mrs Knibbs anyway.

Name: Arthur KNIBBS
Date of departure: 23 August 1956
Port of departure: Southampton
Passenger destination port: Cape, South Africa
Passenger destination: Cape, South Africa
Date of Birth: 12 September 1892
Age: 64 (calculated from DOB)
Marital status: Married
Sex: Male

The following people with the same last name travelled on this voyage: -
Mrs KNIBBS

Ship: CAPETOWN CASTLE
Where bound: South Africa
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Arthur and Florence visited the UK in 1960:

Name: Arthur KNIBBS
Date of departure: 4 August 1960
Port of departure: Southampton
Passenger destination port: East London, South Africa
Passenger destination: East London, South Africa
Date of Birth: 12 September 1892
Age: 68 (calculated from DOB)
Marital status: Married
Sex: Male
Occupation: Retd

The following people with the same last name travelled on this voyage: -

Florence KNIBBS

Ship: EDINBURGH CASTLE
Master's name: Stoakley
Where bound: South Africa

Florence was the daughter of Albert Cecil Oakeshott and his wife Selina ("Lena") Catherine Norton who were married at East London, Cape Province, South Africa on 25 Oct 1893. Albert was born in England and we know that he had a brother Harry who ran a boot and shoe making company in England. One theory is that Albert was "his man" in South Africa.
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From the same passenger list when Florence and Arthur travelled to the UK:

Name: Florence KNIBBS
Date of departure: 4 August 1960
Port of departure: Southampton
Passenger destination port: East London, South Africa
Passenger destination: East London, South Africa
Date of Birth: 12 February 1895
Age: 65 (calculated from DOB)
Marital status: Married
Sex: Female
Occupation: H Wife