It just means that that seppa originally was fitted with others of it's type in a manner that allowed the release mechanism shown in the above photos to pass through from the tsuka to saya. Looks like the release mechanism has been removed which is not unusual. I've seen many that were so the sword could be held in place by friction fit and drawn more easily when required.
The 3 is a part number in my view and I feel it's just coincidence that it shows through.
Regards,
Stu
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