This 50-dollar note was issued to commemorate the 50th anniversary (golden jubilee) of the accession to the throne by Sultan Hassanal Bolkiah Mu’izzaddin Waddaulah. This note circulates normally, but numismatic products are also offered in various combinations and packaging. Courtesy of Chaopiaobaike (钞票百科) and KK Lim (Collectibles Arena on eBay).
According to a press release dated 5 July 2017, Brunei and Singapore have both issued new 50-dollar polymer notes to commemorate the 50th anniversary of the Currency Interchangeability Agreement. Brunei introduced one million notes, and Singapore introduced two million notes, all at face value. Banks in both countries offered sets