Not my specialty, but it is a naval (N) shell case for an "Anti-Submarine" (A/S) spigot mortar device - probably from a "Hedgehog" or Limbo since it has the British "broad-arrow".
Made in 1954 by ECC (Edward Curran & Co manufacturer's Stamp)
Mk 10? (from the stenciling on the case side - not really clear -> "MK XCR" as in Mk 10 Cartridge???)
If you search BOCN with "anti submarine" there's a number of threads on these type of cases etc.
Other members will hopefully elaborate more for you.
OK I got curious myself and did a little searching on BOCN....seems that the "A/S 12" is for the Limbo not Hedgehog.........but not conclusive: