

Here are two photos of a case recently acquired. The case appears to be a new, unprimed empty, and is 57 x 408R86, and in the first photo is shown below a 6-pdr 7Cwt case to give an idea of relative size and shape. The second photo shows the headstamp.
Is this what might be termed the 6-Pdr "long" Nordenfelt ("long" to distinguish it from the 57x306R case which is similar to the 6-Pdr Hotchkiss case)?
If not, what is it?
The Royal Navy had a case in WW1 that was referred to as the "6-Pdr Special" which apparently had a case with the same mouth and rim diameters, but a length of 403mm. Does anyone know if this is the same as the subject case, please?
Thanks in advance for any information.
Roger.