I don't think anyone knows for sure. At least I have never seen anyone come forward claiming they knew the date code format. I have always guessed that this code somehow continued from the 11,22,33 codes on super record crank arms, which I suppose are 85,86,87 respectively. I assume then that 41 is 1988 and 51 is 1989. This is just a hypothesis. My one data point is 1988 marinoni special will full chorus and 41 on the back of the crank. If anyone can add to this there help is appreciated.
1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear