SOCIAL SECURITY FOR MIGRANT WORKERS - BENEFITS - NATIONAL RULES AGAINST OVERLAPPING BENEFITS - NON-APPLICABILITY TO RECIPIENTS OF SIMILAR KINDS OF BENEFITS AWARDED IN ACCORDANCE WITH THE PROVISIONS OF CHAPTER 3 OF REGULATION NO 1408/71 - INVALIDITY BENEFITS CONVERTED INTO OLD-AGE PENSIONS AND UNCONVERTED INVALIDITY BENEFITS - ASSIMILATION TO BENEFITS OF THE SAME KIND ( REGULATION NO 1408/71 OF THE COUNCIL , ART . 12 ( 2 ) AND CHAPTER 3 )