[I've never seen one sell for more than in the $400-$450 range around here.]
Grade I Japanese (Miroku) ATD's sell in that range, for sure - but not one made in Belgium, especially if it's a "wheel sight" gun.
A Japanese Grade III or IV is appreciably more expensive.
Grade I Belgium's typically sell close to $1K in excelent condition - BUT..........
It COULD be that you have a Belgium Grade III there, as you mentioned an engraved trigger guard, too.
A pic would help, as Belgium Grade III's are in the $3K neighborhood.
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