The following picture shows an RST 790 fork from 2010. Anyone knows what the tiny part pointed by the red arrow is for? This part is also present on the RST 777 fork.
From another angle and zoomed in:
It's difficult to find much information about these forks. They lay at the bottom of the RST suspension hierarchy and are coil only forks. They have no adjustments other than preload. While it's not directly stated, but based on looking at the diagram for the 777/790 forks, I'm confident that that part on the left fork leg (essentially a pair of internally threaded M5 x 0.7 holes) is an attachment point for a dynamo.
This would be an external generator (some are called "bottle dynamos" due to their shape), a rotating portion of which can be engaged with the tire or rim. The turning wheel spins the relevant portion of the dynamo which then generates current that can be used to power lights. See this Wikipedia article on bottle dynamos for greater detail. The photo shows a bit of a different style of dynamo attached to a Suntour suspension fork.
The RST forks with associated schematic are shown on pages 76 & 77 of the 2010 RST Catalog with Drawings. Be aware this is a link to a PDF download. It's safe, but one needs a PDF reader to view it.