Thought I would show this as there was a very nice and similar item on a popular online auction site recently. It was a card with several 1939 imperf specimen proofs with ST numbers. For a long time there was uncertainty as to what the letters "ST" represent. Someone later suggested "Specimen Timbres". But a presentation booklet from Waterlow & Sons was found which has the words "Stamps-Timbres" on its cover. It was and still is conventional for the Universal Postal Union to use french as the language of choice. "Timbres" is the french word for stamps.
Waterlow & Sons Ltd were responsible for the printing for the vast majority of the pictorial issues of North Borneo. These imperforated specimen stamps were said to be quite uncommon with about 100 of each value printed. They were often found mounted on a card and the letters ST and number written in Indian ink just above. The numbers should be in chronological order but the numbers for the 1909 and 1911 issues are in the 700s.
Interestingly, STs from the 1897 issue like this 5c were not mentioned in the Sarawak Journal nor The stamps and postal history of North Borneo part 2 1894-1908.
It was quite likely that they were samples used by representatives to solicit printing orders in a similar fashion to the miniature sample sheets of nine stamps. An example of which was shown on one of my posts in January 2012.