SOCIAL SECURITY FOR MIGRANT WORKERS - INVALIDITY INSURANCE - CALCULATION OF BENEFITS - APPLICATION BY ANALOGY WITH PROVISIONS ON INSURANCE FOR OLD AGE AND DEATH - CALCULATION OF THE THEORETICAL AND ACTUAL AMOUNT - SUPPLEMENTARY PERIOD ( ' ' ZURECHNUNGSZEIT ' ' ) - INCLUSION IN THE CALCULATION OF THE THEORETICAL AMOUNT - EXCLUSION IN THE CALCULATION OF THE ACTUAL AMOUNT ( REGULATION NO 1408/71 OF THE COUNCIL , ART . 46 ( 2 ) ( A ) AND ( B ))