hi katana,
it does sound a bit thick for a letter opener, the short shafted pilum that was subject to an
earlier dissertation 
looks like it can take care of your correspondence instead while we ponder the source of your latest weapon.
the two opposing flats on the integral bolster look interesting, could it be for a wrench or tongs to hold it while inserting it in the grip? hard to tell from a small photo. maybe an x-ray of the tang thru the grip by a co-operative dentist, vet or hospital will shed some light. not loose is it? any indication of how it's stuck on, screwed in, cutlers cement, laha or just fibre wrapped & pushed in like a tombak or keris?