A violinist named Hugh Gordon Langton fought in World War 1 as a Second Lieutenant in the 4th London Regiment, was killed at Passchendaele in 1917, and is buried at Poelcapelle in Belgium.
The Commonwealth War Graves Commission, which erects gravestones for and maintains the cemeteries of British Commonwealth service personnel killed abroad, inscribed a six-note musical phrase at the bottom of his gravestone. It is, in the treble clef and in 4/4 time:
Aflat4 crotchet / quarter note
C5 quaver / eighth note
G4 dotted crotchet
It is the only such gravestone with a musical phrase inscribed out of 1.8 million individuals memorialised and nobody, including the Commonwealth War Graves Commission itself, has ever come up with a convincing source for that phrase.
Some hints are that Langton was a Freemason; also, he was plainly an excellent violinist as he studied with two of the great violinists of the late 19th and early 20th century, Otakar Ševčík (well known even nowadays for his technical studies for violinists of all abilities) and Leopold Auer (best known for an unfortunate reason - Tchaikovsky offered him the first performance of his Violin Concerto but he turned it down because he considered it to be too technically challenging).
Unfortunately, I have not found anything about what sort of musician Langton was before he joined the British Army; was he an orchestral or chamber musician, a soloist, or did he teach?
There has been a suggestion that the musical phrase is from a (now) completely obscure popular song, but many people (including me) are sceptical as a classical source would be far more appropriate. A Blackpool Gazette article has some other suggestions; a local composer was inspired to write a work of his own by the mystery.
Edit: The only contemporary mention of Langton that I have found is an advertisement in The Times (1910-10-27):
BECHSTEIN HALL. TO-NIGHT, at 8.15.
VIOLA D'AMORI and HAROLD LOSCOMB.
Mr. HUGH LANGTON. Solo Violinist.
Tickets. 10s. 6d., 5s. and 2s. 6d.
CHAPPELL'S MUSICAL AGENCY
12 George-St., Hanover-Sq., W.
As the link suggests the Wigmore Hall (as it is now) was and is a top-notch venue.