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IMO it should be called "Boer Long Tom (155mm) Gun", with or without the 155mm as this is not essential... since that is a true description.
The French name is probarbly more like (Canon de)"155 Long Mle 1877", the Long is usually abreviated to 155L, Mle is short for Modele.... at least in French period manuals..
Also known as the "155 Long De Bange" as it used a De Bange breech.