The slightly irregular stamp characters together with the unusually heavy stamp application pressure, plus the 45-degree tenon shoulder and tall (for Dunhill) slot says 1975-78.
Mmhm the stamping gave me 70s vibes.
Didn't know if there was a known event that led to the exclusion - but seems like something that would have occurred at Dunhill in the 70s.
Probably dropped the stamp under the table and couldn't reach it.