This cartridge is one of two shown in the c1900 RWS/Utendoerffer catalog as “L.A.” in the “various types” section. The other was the 11.25×54.5R L.A. (GSP48). Both have 12.65mm bases with 14.5mm rim diameters and could have originated from 12mm metallic shotshell cases. The significance of ‘L.A.’ is not known. This cartridge does not appear to have any relation to the series of 6 cartridges produced by RWS and known as ‘La’ for Larsens of Norway (SC13-SC18). Note that this is uppercase ‘L.A.’ separated by full-stops as opposed to ‘La’ which implies it should represent two words “L.. .. A.. ..” .
It could stand for “Lancaster” and it does bare a remarkable resemblance to the 12mm L.A. cases for “Lancaster” (Shotgun) by DWM.