Complex sentence: It is important to recognize that shares were still registered by name in the VOC's register, and that transfer of shares was effected by an entry, witnessed by the company's directors at infrequent opportunities (usually when a dividend was paid).
Split into simpler sentences: It is important to recognize that shares were still registered by name in the VOC's register, and that transfer of shares was effected by an entry in that register, witnessed by the company's directors. Such transfers were allowed at infrequent opportunities (usually when a dividend was paid).