What are the significance of 3 marks that appear on the back of base plates?
I think that this is a production code similar to the seal codes on other sets (e.g. 24R5 for the 24th week of 2015), and you need to add the number from the first dial to the number of the second dial to get a week number, and the Q "dog tag" indicates the production facility this came from. It's missing the production year though, perhaps that was not needed or has been encoded somewhere else on the baseplate.

UPDATE: The year of production is printed in the center of the single digit dial ("11" in your case) with one digit on each side of the arrow. So your base plate was made in 19Q11, which is the 19th week of 2011. This code seems to be on other LEGO elements as well - for comparison here is a picture of a LEGO Mindstorms EV3 tray from set 45544. It also features a Q, and has a "15" printed in the center of the single digit dial (the full code is 17Q15):