From the cameraquest page located here http://cameraquest.com/VivLensManuf.htm I can see the first two digits, 28, mean that this lens was made by Komine,and the third number, the 6 indicates a manufacturing year of 1976,while the fourth and fifth digits being 07 mean the seventh week of that year,and the digits after indicting the serial number of the lens, or number 632.