In 1887 Singer introduced their first safety bicycle, the Apollo.
In 1888 this was accompanied by a semi-diamond frame, with a peculiar small saddle tube, pointing forward.
In 1889 the frame was reinforced with a top tube. The small saddle tube 'grew forward'.
1891 brought the option of a steering lock. The double rubber blocks in the foot rests on the front fork disappeared. From now on it had single rubber blocks. Chain tensioning moved to the rear, from now on the bicycles had a fixed bracket
A new (extra) model was introduced, that made the semi diamond to a real diamond frame.
Other frame tubes, transverse saddle mount, but still the classic Singer safety.
In 1892 or 1893, all above models were withdrawn in favour of a more modern diamond frame.
Illustration below is taken from Slade's catalogue 1893. Who owns such a Singer for real and helps me with pictures?
All these models (except the last one) have their own page on my website, look at the chapter 'sorted by makers' to find them.