Any number cast into or forged into a part, is the part number of the raw casting or forging.
Parts are made in runs of the thousands because it is cheaper to fit dies & moulds and run them to the end of their service life than set a machine up make a few hundred then reset the machines and make a few hundred of something else.
For QC reasons you need to identify which pattern / mould / die a blank was made from.
Thus each one must have a number / mark to identify them with.
The number does not have to mean anything but generally it will be a date code or a sequential , so 96 was probably the 96th set of moulds made from the MAG pattern or some thing similar.
As previously mentioned it is for the benefit of the factory, not the end user.
Numbers that mean things to end users are stamped into / engraved into the parts.
The 96MAG casting is most likely used for several engines and could be bored to take a roller little end bush, no little end bush and several different big end journal sizes.
The foundry would have no idea of these so they can hardly cast an end use code number into the part.
In fact putting a real part number on machining blank can cause a lot of problems down the line.
BSA made nearly all of their own parts and the machining blank had a part number cast / forged into it almost the same as the finished part number.
The finished part number would generally be one numeral less so a piston blank could be 69-4120 which is cast into the piston.
However the finished std size piston is 69-4119, the + 20 piston is 69-4117.
Owners see 69-4119 listed as a std piston then sees 69-4120 cast in his piston so he goes out & gets + 20 rings because he assumes the "20" must mean the next size up.
