Letters and numbers on processors... what do they stand for?

[Castellen]

The "9942" is a date code, it means manufactured on the 42nd week of year 1999.
A lot of IC manufacturers do this, so it's often a good way to tell the manufacturing date of a piece of electronic equipment, given that most parts will be made less than a year before being used in manufactured equipment.

The other numbers printed on there (you didn't quote them) are usually batch numbers used for quality control, and probably revision numbers of the silicon die used, etc.

[Castellen]

The rated speed is only one aspect of the device.

The MC/RC define other things about it.
MC means it's a fully released standard production item, as opposed to the XC prerelease(?) and the SC which refers to customised devices.

The RC means it has the MMU and FPU enabled.

When semiconductors are manufactured, it's near impossible to get a 100% perfect success rate.  There are problems with silicon impurities, manufacturing processes, the way the silicon is made, etc.
So after a silicon die is finished, it's fully tested.  If (for example) the silicon in the FPU section has a problem, the FPU is permanently disabled, and the device is sold as a cheaper 68060LC50, meaning the FPU is disabled.

So in actual fact all the RC/EC/LC variants come from the same die, but due to manufacturing faults, they are de-rated and sold accordingly.
The advantage being is that there is cheaper products on the market for customers who may not require FPU/MMU functions, and there is less wasted devices from manufacturing.