So a quick look on the wubs yielded the following.
Modern 'Nostalgia' Model 15’s
From the mid 1970’s what are often referred to as ‘Nostalgia’ model 15’s were made in Taiwan and then later in China. These were made to the same design as earlier machines with the traditional black cast iron bodies and with the RAF or Sphinx decals etc. They were even supplied in the old style bentwood cases or treadle tables.
The machines built at the Singer factory in Taiwan were identified by the model No. 15NL. The Chinese made 15CH179 is a current model being sold into various African and Asian countries. Although these machine look like the originals, there are subtle differences and they have a poor reputation of being lower quality machines that tend to be very noisy in use.
From singersewinginfo.co.uk