Rifle No 367 (and hence magazine No 367) is shown in the Sals ledger as "To be collected" (with no particular force identfied) a bit of investigation shows that it was used by Derby Police.
According to my records - Enforcer No 367 was 'living' quite close to you in Kent a couple of years ago - it may be that he has your magazine, as you have his.
Why not start a new thread asking for "The Owner Of Enforcer No 367"