Unknown 6 pounder headstamp

[Experiment]

A few days ago i got my hands on this 6 pounder casing but noone seems to recognize the headstamp, so i come to ask for help. its a 6 pounder Anti tank gun casing(57x441r). People have noted how the brass looks different to normal brass and after comparing it to a 25 pounder casing i can say it looks more like a russian casing than a british one.

Headstamp exists of Two serif font " 5 "s at 12 and 6 o'clock both pointing north.





Cheers, Experiment.

 

[Gspragge]

With the groove around the primer hole it is a 'Nordenfelt" style case, beyond that I have no idea.
Though it may well date closer to 1900 but it could have been contracted by various firms.

 

[Experiment]

has the 57x441 been in use since then?