Belted '.50 BMG' ?

[flak88]

I recently purchased the cartridge on the right in a job lot, it's stamped S L 43 and measures 98.5mm in length. Initially I saw the belt and assumed a Boys .55 but on closer inspection the neck did not look long enough and a trial fit of a .50 bullet seemed too tight. I then compared it to the .50 BMG cartridge to the left and all measurements seemed to match other than the belt and position of the shoulder. Hopefully I have not made an obvious mistake but can anyone help in identifying this cartridge ? Many thanks.

 

[LEAFLET]

Hi flak88,
I am not specialized in small ammo but as you said it doesn’t look like a .55 cal to me.
But I don't know wat it is.
LEAFLET

 

[Berle]

Hi flak88,
I am not specialized in small ammo but as you said it doesn’t look like a .55 cal to me.
But I don't know wat it is.
LEAFLET

Might be someone's wildcat? Might have swaged the belt. Can't find anything close in Hawkinson's book below 13mm.