30th anniversary of Civil security.
XXX lat MO i SB (Milicji Obywatelskiej i Służby Bezpieczeństwa) = 30 years Police and Intelligence agency.
Find other Polish stamps commemorating MOiSB anniversaries
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Milicja Obywatelska (the former police force in Poland)
Służba Bezpieczeństwa (the former communist intelligence agency and secret police in Poland)