The number refers to the length of the ‘Virtual seat tube’ in mm i.e. the equivalent length of the seat tube if the top tube was horizontal. (Most road frame are expressed in cm and most off-road in inches)
Your 487 is either a small road frame of 49cm or a large off-road frame of 19"
The L at the end means that it has a Long top tube - effectively meaning that this bike should be used with a straight bar. If it were S - short it would be for drops or an upright sitting position.
The people at SJSC are the best ones to provide you with information on sizes, I would phone / Email them.
